Don't forget to consider the range of your existing rear derailleur. I have a recumbent trike that came with an SRAM X7 rear derailleur and an 11/36 10 speed cassette. When I wanted to duplicate that with a Shimano rear derailleur instead,, it was very difficult to find one that would span that large a difference. 11/32 is no problem to find a number of suitable RDs for Shimano. I found the answer at the Sheldon Brown website where they suggested a suitable wide range RD but with the addition of changing the "B screw" to a longer one.