more or less suspension in the rear as manufactured doesn't effect geo because a lever (the link) is used as a multiplier on the shock. A longer fork always will, so it will just be the usual hta, sta, reach, bb height etc. Do you want a better climber or a better descender? Longer will be a better descender and more stable at speed. The cost is less control on steep climbs, less nimble, and despite being slightly higher the bb will actually have less clearance on drops etc because of longer wb.
I should add that its little to do with riding, and more to do with getting a bargain.
I notice on ebay,wiggle,CRC etc that usually have discounted forks, and sometimes you can be paying half the usual price. But you might be limited to 170mm and i was looking to take up such a bargain if it came up. The fork i have is the fox 36 rhythm, but i see anything up to fox 38 performance, lyrik ultimate, fox factory going at very good prices and looking to upgrade. So knowing if 170mm wil be ok and not mess up the geometry too much i could go for it.