Here’s a comparison. M35 is on the left, M34 on the right:
Per product descriptions M34 is 13.3 x 5.6 x 2.7 inches and M35 is 13.2 x 5.6 x 2.8 inches. So M34 is one tenth of an inch taller. And M35 is one tenth of an inch thicker in the torso. Or to be more precise, M35 is just more bulked all the way around, i.e., in the chest, arms, legs. Basically M35 is thicker and a smidge shorter.
One thing to consider is that from what I’m hearing from other kitbashers, M35 is so bulky it’s hard to find clothes and armor that will fit. M34 might therefore be a bit easier to find already produced clothing and armor for. But even with M34 I may have to commission an artist to make those items. Although perhaps when all is said and done that may prove the wisest thing to do anyway…
Minsc is huge. So either body can work. But I’m thinking that maybe the tiny bit of extra height that M34 has will, overall, be preferable to the massively thick frame of M35…?
For reference sake, here’s a gallery of images I’ve found for Minsc. Most of these are official WotC, bottom right is fan art. Some have him freakishly huge and bulked. Others have him muscular but more normally proportioned:
I think in the balance I’m leaning towards having him as tall as possible but still with a lot of brawn, which is what M34 gives me.
One Sixth Baldur's Gate 