Not sure but I can give you a somewhat edumacated guesstimate, as I own a Bass 400 (non+) and got to crank up the combo a little this evening.
If it were me trying to pick between the two, I'd probably get the Prodigy as I really like what the 400 delivers, and the Prodigy is a 250-watt tube head in a smaller package. I think the M6 would give more clean headroom, but what are your goals? If primarily clean with solid lows that don't run out of steam, go with a SS amp like the M6 or probably the M9. If you like what tubes do when they get loud (natural compression, OD the output stage, etc.) then go for the Prodigy.
If I put just the M6 amp up against my 400 I think through the same cab the Carbine would be louder and have more low end. I power an Ampeg PR810H with the Mesa; I'd like to hear the differences myself and will probably do a head-to-head test some day just to see what happens.
Additionally, this is my first experience with 12's (and they're NEO's) and I'm very impressed. Mesa's 212 cab goes deep, has highs, sounds smooth and balanced, and gets pretty darn loud - far more than what it would ever be called upon to produce for the environments I'll use it in. I dare say it's solved my amp/cab gas for probably a while. I can also see why 12's are the new 15's. I like dual 15" cabs, but I really like the sound I get out of this beastly little combo.
Anyway, that's my biased opinion for you.
Raz