No matter which multi-effects unit you choose, my input would be to make sure it has a bypass feature. If that's not important to you, then by all means ignore me. But I wanted to at least share my experience and mention it.
I think that's great advice.
Most multi-effects pedals add a certain amount of digital hash or hiss to the signal chain unless they're able to be bypassed, but there's one solution to consider: A true bypass loop switchbox. With one small box, you can completely switch out the multi-effects, and if you like, even switch additional stuff in and out of the signal chain.
This place does a nice job:
http://www.roadrageprogear.com