How does IcAros perform using UAE compared to Amithlon using UAE?
Amithlon is faster, mainly because you can use OS friendly apps directly without UAE. If you run UAE, both are pretty much identical (assuming there's UAE build which uses the x86 code under Amithlon, I have no clue if it does).
If you're only going to use UAE then you shouldn't bother with neither. Neither have UAE as good as WinUAE. Seriously, you should try
suspending the windows box rather than powering it down. ;-)