Given like for like quality I would opt for the bigger size. Makes sense in regards to better heat dissipation, modulation, wear, etc. The only downside I can imagine is slightly closer proximity to things that might wack it. 1cm ain't much though.
180 v 160 makes a massive difference in my experience to the total braking power.
I have 160's f&r on my main xc bike and my son has 180 160 on his. The difference is really noticeable when I ride his set up
I guess I have been jumping on different machines all my life and find it hard to notice I am squeezing harder or such just like that, but that is more to do with mechanical performance than pure rotor size.
I have a huge floating 203mm vented rotors with the setup on the Prince Albert.
That is a lot more braking than I will ever have traction to use.
What for?
I see the point of bigger discs on front, but I am sure a 140mm vented disc would give me all the braking I need, with less weight.
It stops quicker. I'm not being fasicious, and I certainly accept the points made about feel, limitations of tyre grip etc.
It's a fair comparison in my case, both bikes are fitted with m-760 xt brakes, and the bigger front disc one just stops quicker.