Crank lengths. Any thoughts? Is it primarily leg length or does hills/flat come into the equation?

I have long legs and I prefer longer cranks. Some people prefer to spin more and have shorter cranks. I have read some very convincing evidence that crank length should be related to leg length (mostly femur). Some people disagree.

It is primarily personal choice.

This is supposed to work out what you should use based on your leg length:
http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

I’ve got 175’s and 170’s with a 32.5″ leg. The 175’s are easier to haul up a hill at low speed, but the 170’s are definitely easier to spin fast.

Don’t forget to adjust seat height to suit!

I’ve got exactly the same as simon (my 175mm are on an SS, 170mm on the ‘normal’ bike), for the reasons already stated.