to be honest
you'll probably find ableton a real pain in the ass after using cubase
but if you stick with it you'll begin to see the benefits
and creatively it might inspire you.
but you can't beat the cubase arrange window , it's the best layout of all
even logic doesn't quite beat it ( and i use logic now )

anyway i've just got ableton 5
and it's wicked but using it to play external midi hardware is a bit sloppy
and it's midi sync at the moment is crap too

4 was better , 3.4 was better still for reliabliity and tightness
but 5 has a load of cool options and the arrange page has been improved.
definitley worth checking out
download the demo and try that first before you buy

www.ableton.com