Okay, so I own the MicroKorg and I absolutely love it and would not trade it for anything. And on that note, get the MiniNova. I just watched a video on it and from what I just seen it is a lot better than the MicroKorg - On terms of Presets and functions - I did not watch long enough to evaluate the ease of creating sounds.
Anyway from what I have seen, the Novation gives you 128 slots to save your own patches, the MicroKorg on the other hand, you have to save over a Pre-existing sound - and If you accidentally saved over a sound you wanted to always have then you are out of luck, because the only way to get that Preset back is to Factory Reset it - which all work that you had will be gone.
And sound wise from the demo's I heard, I honestly like the sound of the Nova more, while it is based on preference, I really enjoyed the sounds of the Nova. The MicroKorg really excels at making Nintendo music but when it comes to other genres it just doesn't sound as good as it could be - I mean it could be me and my capability, but I don't have the problem on other synths. The Microkorg also does pretty well for Trance, but when it comes to making hard leads, it just doesn't do it for me. And just from the presets on the Nova, I know I would be able to create better sounds on it, however it could never beat the Nintendo sounds the micro can make.
I cannot elaborate on the vocoder as I don't use it myself, but the Mic that comes with the Nova looks a lot sturdier than the Microkorgs gooseneck Mic.
Now again, that does not mean that the MicroKorg is bad, I absolutely love it, and if you went that route you would not be disappointed in it.
What you really need to do is to keep on watching videos on both synthesizers and decide what you like more, because it is really based on preference
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Now, just food for thought, another synth that you can throw into the ring to consider is the Arturia Minibrute. It is a full Analog synthesizer, no menu diving, 100% hands on. And again it is all about personal preference, but in mine it wipes the floor with the other two in terms of hands on capability and sound. It is also $500 as well. I mean you get a lot. For a few examples just watch these videos:
The Marc Doty Review series:
http://www.youtube.com/watch?v=s1DCxaTE0mg
Adrian Utley
http://www.youtube.com/watch?v=X614aeTzq7k
It's just another possibility to consider, Any route you go will not disappoint!
Good Luck!