Do you have any trouble climbing in your easiest gear? If not then you really don't need the 27 on the back and you can get by with an 11-25. If you do want easier gears then go with the 11-27, you will have to purchase a Shimano cassette though. Shimano and SRAM cassettes have the same spacing so it will work fine. I am currently using Shimano cassettes on both of my bikes which are both SRAM equipped.