+1 personal preference.

Spinning, low gears, I'd go shorter cranks. This for trackies, flat landers, riders who wonder why anyone ever made a 13T, forget about a 12 or an 11.

Power, higher gears, short power hills (90% of hills under a mile) I'd try longer ones.

When making a change (or trying a change) I'd go with a 5 mm change. 2.5 mm is not a big change, and if you want to really gain the benefits of a different crank length, really go for it. No half-hearted changes.

For me I dropped my saddle 5 mm when I went 170 -> 175. I initially tried a 2.5 mm drop and experienced knee pain within minutes (I have fragile knees). The full 5 mm and no problems at all. I had to be careful not to push too big a gear - with long cranks you feel like superman. I'd big ring a lot of hills that I'd normally small ring. It's the power hills, normally taken at 18-22 mph, which really get affected by the long cranks. I find I can roll a significantly larger gear on those little rises.

Finally I'd give it 20-30 rides before making a judgment if you go long. Fine, I judged my long crank preference in one ride (non-sprint speed on mountain bike with 2" knobbies, suspension fork, etc with 175s was higher than sprint speed on road bike with 170s), confirming it when I put 175s on the road bike (sprint speed went +10 mph on same stretch of road, about a month between rides, this when I was extremely heavy and extremely unfit).

I find longer cranks get some getting used to because you pedal so much slower. Your feet travel at about the same speed, but you're moving them in bigger circles. Math people, pipe up. Since I move to longer cranks after the season ends, or I keep them on most of the time during the season, I am more at home with long cranks. I haven't really figured out a time-pressed minimum adaptation time for longer cranks.

I think this adaptation time is critical. I put off trying longer cranks for two years because I had a very narrow window in which to switch cranks (Oct-Nov, to test and adapt before March, or, if it failed, to return to the shorter cranks in Jan or so). However I'm really glad I made the move to longer cranks.

Going to shorter cranks is much easier, but you lose the fluidity of weeks/months/years of riding the shorter crank. I find it takes 1-2 weeks to get 99% of my short crank game back. I've never gotten back to my 115-120 rpm averages though, it was always more like 90-95. I found myself speed limited on the track when I moved to the 170s on the track bike on Wednesdays. On Sun/Mon/Tue I did hard rides on the 175mm equipped road bike (race, hard group ride, race), and on Wed, on 170s, I couldn't spin well. I finally decided to go to 170s on the road in order to make my track pedaling more fluid. I won a few fast races (Keirin) right away.

Then I crashed and couldn't ride for a while, but that has nothing to do with crank lengths

cdr