2 points:
1) Advantage of 170mm (and longer) is that there is a greater choice out there (and so you may find it easier to keep costs down if that's important). If you are thinking that you might go fixed gear in the future, you may want to consider shorter cranks - there is a good thread somewhere on LFGSS with lots of BB height / crank length combos that people find to work OK on fixies.
2) Have you got another bike that you ride regularly at the moment? If so, you may want to stick with the same length cranks as what you've got already as you will be used to the feel of that.
2 points:
1) Advantage of 170mm (and longer) is that there is a greater choice out there (and so you may find it easier to keep costs down if that's important). If you are thinking that you might go fixed gear in the future, you may want to consider shorter cranks - there is a good thread somewhere on LFGSS with lots of BB height / crank length combos that people find to work OK on fixies.
2) Have you got another bike that you ride regularly at the moment? If so, you may want to stick with the same length cranks as what you've got already as you will be used to the feel of that.