I'd strongly recommend the Roland MC series sequencers. They're in their umpteenth generation and all but bulletproof. I sold the old MC-500 and customers were always wowed by its robustness. They're still cut from the same cloth.
I bought an 01W/fd partly FOR that excellent sequencer; the nearest equivalent, sys-ex patch storage n' all, is an MC sequencer. Boy, if Korg would only put just THAT in a stand-alone box...
I personally find Yamaha sequencers fussy and the Alesis MM-8, while a good unit, doesn't have the horsepower to meet your need here. Its become more of a techno sideman than a do-it-all unit. It also has no disk drive, which you WILL want.
If you can use a laptop for this purpose, great; that would be the ultimate way, but they're much more delicate than the dedicated sequencers mentioned here.
Just set up that ol' tradeoff algorithm of need vs. available money vs. how much jostling the chosen wunderbox will undergo and what else you might need it for if you travel and play live. Check out ugbm.com and
www.roguemusic.com for good 2nd hand items. $400 is a typical round figure for these units.
Then get disgusted when it all blows up during a power surge and go back to the accordion.
HP Huey