As usual, I have an opinion about this sort of stuff.
The best way to look at this is by comparing surface area first.
the 35mm frame has a surface area of 864mm2
the 6x6 frame has a surface area of 3600mm2, or 4.16 times bigger than 35mm.
So it means you can use a film that has 4 times the grain of 35mm.
Now, there is the resolving power of the film that is provided by the manufacturer.
TMX 100 has a resolving power of 200 lines/mm for high contrast subjects and 63 lines/mm for low contrast.
TMY 400 has a resolving power of 125 lines/mm for high contrast subjects and 50 lines/mm for low contrast.
Since you're going to shoot in low light, contrast is likely to be pretty high... so lets use the high values.
When you calculate it, TMX 100 / TMY 400 gives you a ratio of 1.6 pretty far from the 4 times the surface of the film.
So you should still get finer grain using the medium format camera with faster film.
I know this is a Mickey Mouse way of calculating it and that I should be using RMS Granularity and stuff, but I think that in the end the reasoning is sound.
And even if I'd square root the difference between 35mm and 120, it would still show that 400 iso film will be less grainy on 120
does that make any sense whatsoever?