Well, both based on a gut fill and on calculation of damage dealt per 3 tiles of charge - GM is stronger.
But I have tried several times with GM and Mits- and found that statistically I win more often with Mits. This is indeed caused by Vela. But that is what we have now.
As for the speed - Boldtusk charges with 9 tiles (talents, suit) and Mitsuko after Grazul ultimate ability and her element link (mana speed) also charges with 9 tiles. So specifically in this offense team and against Vela flank - Mitsuko seems to be stronger.
Obviously GM is better in defense. The only issue - his class Rogue - same as Kageburado who also is in defense team.
And GM is better in 3+2 teams. However I think 3+2 is preferable when you have almost all heroes 5* for attack. This is not my case yet and I play mono, thus this is the team that the candidate had to fit into.
Anyway - I may be mad, but I chose Mitsuko… Hope Vela will stay on the flank of a green center