some guitar shops that sell single strings don't take care of the strings, and they often lie there for ages, slowly deteriorating. some of the single strings at swee lee are in pretty bad shape. this isn't meant to be a cheap shot at swee lee, just my neutral observation.
if i were in your position, i'd buy from ranking (opp. swee lee). they're single strings are kept in their original boxes and seem well maintained.
oh, and btw, if you're interested in something different, check out zachary progressive tension strings (just google zachary strings). i started using them recently, and i like the feel they have.