Here was my take on this topic about three years ago. There were fewer options then, but I know I made the right choice.
Thoughts on the Alesis Micron synth...
First, to address the Micron vs. microKORG battle that wages on, I stand by my assessments made in that thread and elsewhere. Here's how I see it, simply stated: the Micron is a much more capable synth and feels much nicer to play than the microKORG, but the learning curve is a bit steeper because you have far more going on feature-wise. The microKORG is a simple, easy-to-use synth with baby keys and easier to get going on right away. While I agree with Nick's points about the microKORG's longevity and ubiquity, I can't help but view it as the Squier "Stop Dreaming, Start Playing" Starter Pack of synthesis. (I'll probably end up with one someday.)
If you're interested in complex synthesis and want to really nerd out, the Micron would be a much better fit; if you want to make cool sounds and don't give a shit how, the microKORG would be the way to go.
This guy's videos completely sold me on the Micron:
Gtechture on YouTube
Having said all that, my vote for you would be to get something to interface with the iPad. After buying my Moog LP, I sold the Micron to fund other things because I wasn't really using it for anything other than Moog sounds at that point and it was terribly underutilized. I had been vaguely looking around at compact hardware synths again to bring some polyphony back to my setup but never settled on anything.
When I saw the email from Moog about Animoog on the morning of its release (coincidentally my birthday), I went and bought an iPad later that day. I could never really justify the cost of an iPad, but here was an opportunity to buy a portable device to use not only for Animoog, but for ReBirth, iMS-20, Sunrizer, and other great soft-synth apps. For a bit more than the cost of about one fancy new hardware modeling synth, the iPad was well worth it and it's become an important part of my setup.
I've since purchased the $30 Apple Camera Connection Kit to use my Novation ReMote 25 LE with the iPad, but I've been waiting for that damned Akai SynthStation 49 that aen linked; for me, that's going to be a dream.
A quick note: if you decide to get the Line 6 Midi interface, be sure to get
version II! The one aen linked is slightly cheaper, but it doesn't support iOS's CoreMIDI -- meaning you can only use it with their crappy synth apps. Also, the Alesis iO Dock may look tempting, but Alesis seem to have really fucked their userbase by not creating a version that works properly with iOS 5. It would be an amazingly useful piece of hardware if not for that.
Let us know what you decide!