Have had both. Would choose Livewires any day. UM2s had recessed highs. However, Livewires are very revealing of source and can be perceived to be dry/analytical, whereas UM2s were definitely musical.
I realise your age may provide budget limitations but I'd hold onto the SuperMicro IV and go Livewires - you'll then hear all the SuperMicro is giving you.
If you can't afford that, and these two are your only options, you'll find the UM2s (with appropriate tips) to be a very effective, fun, musical phone. Me, I'm more at the detail end of the detail/fun spectrum (http://www.head-fi.org/forums/f4/sou...al-fun-273599/
As to ear growth check with your audiologist but I'd suggest you are probably more likely to have growth changes in your ears over the next few years than older folks like me (43!
). I read on the forums that I may outgrow them in as little as 3 years but my audiologist assured me that it was more likely to be 10-15 years unless I was having teenage-like growth spurts