150s or 155s?

I am thinking of buying a new set of profile cranks for my unis. I have a 24 x 3, with gazz tire, and 170mm profiles, and a monty wheel with 145mm profiles. I was thinking of getting some 150s or 155s to be able to use on both my trials (super technical stuff) and my MUni, (less technical but would be faster) Is this a good idea, and which size is better. Oh yes, and i’m pretty sure that the cranks w’ont hit the ground on the monty, since my friend once ran his 158mm monty cranks on his Trials uni, without any trouble.

Thanks alot,

-Ryan