I thought I'd put up a post on portable music making options currently out there. Its been a few years since the iPhone and iPad explosion and there are a significant number of other devices hitting the market. Obviously if you are only in the phone arena, the market is very diverse and mixed between Android, Apple and some notable others (Microsoft, RIM) etc.
If you are trying to make music, though, at the time of this writing its mostly a one company show still - Apple. The reason boils down to one thing - latency. A simplistic example would be with a simple piano app and latency would show itself in the delay you encounter from hitting a virtual "key" and actually hearing it.
I'm going to talk mostly about tablets for selfish reasons - I find phones still too small to really develop music on. I like having a little bit of room to manipulate sounds and the iPad (full size model) is about right sizewise for creating music.
The alternatives are the Android tablets and new tablets from Microsoft. In the case of Microsoft and Android, the CPU specs aren't bad at all, the screens are ok but at the operating system level there is still....latency - which is a show-stopper for music making. All tablets have pretty good music playback capabilities but when it comes to processing events and creating notes somewhat "immediately", Microsoft and Android vendors have work to do. Android is supposedly addressing this with each release but it will probably take time.
So, if you are looking to make music on a non-Apple tablet, you are essentially a contrarian. For reasons of your own, you may hate Apple or are looking to squeeze a few dollars. The best current options in non-Apple would be "tracker" style music makers or systems where you define patterns and play back the resulting sounds. Here latency would not be an issue. Examples might be Sunvox and/or Mixtikl (two great options on Apple that have ported to other tablets - Android, not Microsoft yet).
The options on Microsoft are fewer but with the "larger" tablet, you essentially have a full Microsoft OS which will run desktop apps. The problems are that I believe there is still latency in their drivers and if you are using a non-touch app, you are essentially just using it like a laptop. A much better option would be to just use a laptop if that is your interest.
So if you're into portable music making and have plunked down the cash for an iPad, where should you put your money? In my opinion, the two most important factors music-wise are the CPU speed and the storage. WAV files are very large and the more samples, mix downs and DAWs you buy, the more space you gobble up. You might want to forego the "cellular" option and put your money into more memory - get the 64 GB or even the 128 GB model. I find the 64 GB to be pretty good for my own very active music making.
Due to the sandboxed file system on tablets, you will find yourself copying WAV files around from app to app (or using AudioBus as I've written about). You'll find that your battery life does go down quite a bit when you multitask and particularly using CPU hungry music apps.
The upside of using the iPad is that the cost per app is a fraction of what desktop plugins cost you. You will save tons of money on software and can build up a huge collection of synths, samplers etc. very cheaply.
Many apps go on about their support for Midi keyboards - and if you use a very low power keyboard with the camera connection kit (or a dedicated iPad keyboard), it will work pretty well. However, I find that once I connect an adapter, usb cable and keyboard to the iPad, it no longer feels "portable". I'd rather connect up to my desktop in those circumstances. I prefer to use the unique multi-touch and aftertouch capabilities of the iPad itself in making music.