The difference between 165mm and 175mm about 0.4" and if you think that you can feel that difference you are star material and should be looking forward to the TDF. The 175's give you a minuscule more leverage and that is a good thing. I'm the same general size as you are and I've been using 175's on everything for a few decades and I tried everything from 165 and up on the same advice as you're getting here. I don't and never did spin [MENTION=175934]120[/MENTION]+ r's all day long and you prolly don't either. Watch the TDF'ers - neither do they.

More importantly if you're 5'7, 30 inseam and have a 49cm roadbike you should be looking for a bike that fits before you worry about the cranks. A traditional frame configuration of 53-54cm would be far more likely to provide you with a quality experience. I'm talking real world go for a ride have a good time fast as you can and don't hurt yourself kinda experience.

prolly not JMO but YMMV and it's your back and neck

besta luck