The only real way to decide is to try different length cranks and see which you prefer. Don't believe any recommendations you read - it's too personal. Some people don't notice 5mm or more difference, while others notice 2.5mm or less. 170mm is the easiest and cheapest to get...

FWIW, I've tried 165, 170, 172.5, 175, 178 and 180mm and decided that 172.5mm was the size for me on all bikes except the fixed wheel. This has 170mm to make spinning down hills less traumatic - it felt like my legs were about to be torn off with 172.5mm. I couldn't climb comfortably with 165mm, and never hit the pedals with 170mm anyway, so it was an easy choice for me to make. If you have much longer or shorter legs, or are used to longer cranks from riding a MTB all the time your most comfortable length may well differ.