Tighter gears mean each of the 9 speeds on the rear cassette are closer to each gear sizes, meaning for smoother shifts. Where as larger gear sizes like the 34t, will help you pedal up hills, but the shifts will be a bit harder.

I personally chose the 34t chain ring for help on going up the hills. There are a few times I hear a clunk when switching some gears, but to me it's worth it when climbing. Now if you don't have that many hills, or strong enough, you may not need it.