Originally Posted by
Bingod
I suggest buying the cheapest cassette, that way when you realize +2 teeth doesn't make enough of a difference, you wont have wasted so much money when you buy the 11-42.
But there is fair chance the 42 tooth will not work with current derailleur and chain. With the 36 tooth you are probably 95% likely to have no problems with current setup. The 34 to 36 tooth is about 1/2 of a typical gear step lower, and maintains decent spacing between your other gears.