I am building a Soma Double Cross with 105 STI and a compact double up front. I was going with a 12-27 10 speed cassette to begin with, since it will be a commuter. If I ever want to tour I thought being able to put on a 12-34 might be nice. So before I buy the RD is there any down side to putting on a Shimano SLX instead of a 105 rear derailleur? This would allow me to have a commuter wheelset and a touring wheelset with different ratios.