I am considering getting a new crank for my CX bike and I was kind of drawn to the Shimano CX50 crankset.

The issue is that these are only available in 170mm and 175mm arm lengths. Both of my current bikes utilize 172.5mm arms...they came stock on the bike.

This begs a few questions.

If I switch arm lengths, will it be noticeable? To me, 2.5mm either way seems negligible, but I could be all wet.

If I switch, does it make sense to move up or down in terms of lenght?

Or, should I just find a crank that I like that comes wtih 172.5mm? I think the CX70 is offered in 172.5mm, but I think the it is substantially more money.

Any insights into this issue would be greatly appreciated?