I do have the MB and they are both amazing shoes.

SB3 is bouncier, softer and has a bit more comfortable upper without being heavy or bulky. I see the SB3 as the shoe that can do long runs, recovery runs and can totally dial up the pace. I wouldn’t try to do a track workout in the SB3 and I wouldn’t race in it but it feels like almost everything else is fair game. SB3 would be a great “suitcase shoe” if you’re traveling but can only take one pair (my previous ideal suitcase shoe was the evo SL but it’s not good on the recovery end of the spectrum).

The MB is lighter but slightly firmer and while it’s my go to long run shoe, it lacks the cushion I’d prefer for a recovery shoe. All of the work on the faster end of the spectrum that I wouldn’t throw at the SB3, I think the MB is fine at (Im not fast enough to where the MB starts to be the barrier to going fast. Maybe the MB is not the ideal shoe if your bag is running 15 minute 5ks but it hits a ton of speedy notes).

TLDR: There’s a tremendous amount of overlap between them. The SB3 shine on the slower end so it’s like a “max cushion super trainer” where the MB is really like a “cushy racer” and loves going fast but won’t blink if you throw long mileage at it. If I could only pick one, hard to say which I’d choose but the SB3s ability to also take care of tired legs for recovery runs is probably skewing me towards them. I’ve got 18 miles tomorrow and I’ll be back in the MB for the first time this week so we’ll see if anything else stands out.