I’ve played a game with a friend who had 12mm tanks and infantry versus my 10mm and there is a very noticeable difference. Not deal breaking by any means, but as others have said, there is currently a greater choice in 10mm.
I have H&R 1/300 [6mm] and my main opponent GHQ 1/285 [6mm]; fielding the two sizes in the same army is [would be] problematic as the 5% difference is noticeable when side-by-side but not when fighting against another army. I would imagine that 1/144 [12mm?] to 1/150 [10mm?] at 4% differential would equally be an issue, other than for infantry where a 5% difference in height is tolerable, given variations in the height of individuals.
What would be helpful would be to have some common understand on sizes. If 1/300 is 6mm, surely 1/150 is 12mm and 1/100 is 18mm. Or should 1/300 really be 5mm? I think each manufacturer has their own view, which explains some of the variation and why the only way to be sure is to put them up against each other or ask someone their experience.