Choosing the best phone for mobile learning can be quite a challenge. It also depends on what you want to do with it and how easy you can get used to technology. The amount of features you want to use will also allow you to purchase a low or high cost smartphone or mobile device.
Let's say you want to explore mobile learning, these are some of your options.
First of all if you are a tech savvy person you could go for a less expensive device which has all the options of the iPhone and more, for the iPhone costs a lot, yet is similar to many other mobile phones that are cheaper. The iPhone is above all an incredibly easy phone to use (very intuitive in its user interface), so for those not that comfortable with technology, the iPhone is the way to go (although it is expensive). If you do not want to pay the cost of an iPhone, you can also go for an iPod Touch, which is much cheaper and offers all the iPhone options, except phone possibilities (but the phone possibilities are not essential for mobile learning).
The same is true for PDA's, with these types of mobile devices you have a range of possibilities, but not the phone possibilities, this again decreases the cost. You can also simply use a MP3 player, as these will enable you to share audio material (e.g. language learning or gather all the songs of birds in the wood and determine which bird it is...)
What can you opt for in a high-end phone or mobile device?
When deciding on a phone, it is good to know what features are most important to you and what type of mLearning you are envisioning, e.g. if you want to use location data to tag your pictures for field trips or similar, GPS will be necessary.
Depending on these features you can make a selection of phones and then compare them in one of the review sites (or a couple of review sites, sometimes those sites already have a 'best off' list: http://reviews.cnet.com/best-cell-phones/ , or a funny geeky one: http://www.testfreaks.co.uk/mobile-phones/ ). In any case it is good to look at which devices are on your local market and then look at (and compare) the specifications of your short list of mobile devices you want to choose from. Go for sites that offer both expert reviews AND peer reviews, because it is the user that is important, not the geek.
Personal criteria that might influence your mobile device purchase (basically what you do is check what you feel is important to you, the more possibilities, the more your device will cost - most of the time):
Camera quality: how many mega pixels does the camera have (the more mega pixels the better the quality of the pictures taken with it)? this was critical too me, as I like to record high quality movies and pictures for later recall (visual person)
Memory extension: if you use a lot of big files (movies, audio, podcasts...) it is a good thing to look at phones that have a memory slot (most of the time it is for mini SDcards), this offers you the chance to have access to more files.
Wifi possibility: if in doubt, I would certainly go for wifi enabled phones: wifi enables you to connect to free internet, which will save you a bundle especially when being abroad.
Bluetooth: this might be interesting if you want to connect to other devices without using a USB connectivity cable. It is also handy for health check-ups, they use bluetooth for instance for wireless interaction between a blood pressure meter and the software to analyse diabetes.
Phone capabilities and specifically quad band connectivity: only important if you travel internationally, because if you only have access to - say 2 - phone frequencies, you will likely not be able to access in some continents.
Mail access: is there a possibility to exchange between your mailbox and the mobile? Or is it of interest to you? If yes, look for that specification.
In addition to that one: synchronisation with outlook: only necessary if you work with outlook, this enables you to exchange mail and your contacts between your desktop and mobile phone.
Office possibilities: this enables you to look at office documents (depends again on your need).
Social media widgets: does the device offer easy to install social media widgets (e.g. facebook, flickr, twitter...). Having this function will enable you to quickly personalize your phone to your own social network.
GPS: that is increasingly important for geo-located learning as well, in those cases the GPS will know where you are at and offer you nearby information (shopping malls, geek stores, museums... linked to your location). But of course also easy to find your way around unknown territory.
Applications - can you add software that is build for your mobile phone, to your mobile phone? To me, being able to add extra applications to a phone is very important, as applications offer a wide variety of possibilities not necessarily integrated in the basic features of a phone (e.g. speech to txt, keeping track of your walks, comparing prices of products across various supermarkets and looking for the cheapest product in range, recognizing buildings...)
Touch screen or not: again this is very personal, some like touch screens, others do not (with touch screen you cannot write a quick sms blindly or from the pocket of your pants for instance, but on the other hand it offers immediate contact with what you see).
Screen reflection: if you want to use your phone outside regularly, it is a good thing to check readability of the screen when you are outside, standing in the sun.
Battery life (thanks to Nick Short!): this is pretty important if working away from a power source for example on a field trip.
Now with all these above mentioned features, it is clear that they are only interesting if you really want to explore them, otherwise it is better to buy a simple 'intermediate' smartphone which offers access to internet, phone capabilities and basic note taking (and the price is much lower than high-end phones). With these 'simple' phones you can already exchange pictures, movies, and discuss what these audiovisual materials you gathered with peers. From a learning point of view, even the simplest phone can add to a learning process.
What mobile operating system to choose?
Let's say you go for the more high-end ones, at that moment the operating system of the phone becomes very important (apple mobile operating system, windows mobile operating system, android OS, Palm OS, Rim or Blackberry OS, Symbian OS (the last one is the Nokia operating system)). Of all the Operating systems, the blackberry is the most restricted one as it needs to be connected to a central Blackberry server (rectification thanks to mLearning-world.com (who work a lot with blackberry comment - see below: the Blackberry's do have third party software that allows synchronisation with e-mail without having to go through a BB server).
There is also the operating system to consider when buying a new phone. Btw: always buy a phone that is not the first generation (these have many childhood diseases), always go for a second or third generation of a phone (e.g. do not buy the first android phone, but buy the second version they put out).
Both iPhone, Windows Mobile and Android offer 'application market place', and for the android many of them are for free. For instance if some application would be useful, but does not exist yet, android offers an open SDK (= an open development kit, which is similar to easy building blocks to build mobile software that the mobile device can run). And - thank you to Jra! - the iPhone also has a SDK kit which is easier to use than the Android one (but the Android SDK is open source, so it is again what you like best as a user or developer).