Both the Micro and the Alesis are good enough for starters as they're very cheap, compact, and have enough capability to keep you entertained for a while.
Eventually, though, if you stick with it you might find them a bit limiting. The thing about synthesizers is you're either using them simply to use the sounds they have, or you're using them to create your own.
Both those synths will let you down in the creation department, but with the microkorg coming out on top with more superior editing capability than the Alesis.
If you're interested in cultivating your own sounds, you'll need to look into something that's programmable down to oscillator types, LFO, filters, etc...
Also note that neither of those synths are true analog synthesizers, but are analog "modeling", which is the equivalent difference between an actual Mesa-boogie tube head and a software modeling of one. So, keep that in mind if you're focused on specifics.
Depending on your price range, there are some very powerful synths out there that have enough depth to allow you to learn and grow with just that single synth, never fearing hitting a dead end in terms of capability and flexibility.
I can recommend some if you'd like, but you're generally going to pay upwards of 1,500 USD on even the lesser models.
Let me know what your price range is, and I'll see if I can't come up with something that fits.
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