Hey Tony.
I've tried both, and I just bought the MB500 (if I had a bit more cash it would definitely have been the fusion).
The tonehammer looks really cool aestheticaly to me, but really it didnt seem do a heck of alot tonally. I experienced the same thing with the Tonehammer pedal. This isnt necessarily a negative, at the time I wanted something relatively flat without massive extremes of EQ that I could just tweak for the room as I use a Tech21 VTbass for the majority of my tones.
The fusion is a different beast it sounds warmer and more 'ampeggy' than the MB500 but its not a night and day difference as they both have the 'GK growl' that i'd always heard about but never knew what people meant till I heard it. The drive sounds are great too.
TBH in my opinion the TH500 wasnt that impressive, not a bad bass amp but is pretty subtle to my ears and the fact is that for roughly 1/3rd of the pice you could just get the pedal and wack it into the effects return of whatever you currently use. The fusion has a wealth of tonal options and just kicks @ss in my opinion. I'm happy with my MB500, but the fusion is a bit smoother - however with my VTbass/MB500 combo i can get very similar sounds to the fusion.