Count the teeth on the largest cog of your current cassette. If it has 32 teeth and you sometimes find yourselves wanting a lower gear, go ahead and buy an11-34. If your happy with your current gearing, then buy the same cassette, 11-32 or 11-34. If you never use the largest cog, whether 34t or 32t you might consider a cassette with closer spacing. We use and 11-32 and there are times I'd like the 34t cog it things get really steep, but the spacing on the 11-32 works better for us. I think it's all about the type of riding you do and how strong a team you are. If you are replacing your cassette you should put on a new chain as well unless your chain is very low mileage.