The MPX500 is an awesome unit, much better than the M-One IMO, and if you do a search on it you'll come across a comparison that Brucie Blue Bear did between the 2. I partly bought the Lexi on his recommendation. The MPX 2oo is great too, just minus the tweaks of the 500.
I also REALLY REALY like the Roland SRV3030. I like the Roland reverbs a lot, but this is more of a dedicated reverb unit, the MPX is kind of a multifx thang with a whole bunch of other stuff. In your situation, I'd go the Lexi
El Linko