To me, Star Wars: Knights of the Old Republic seems like a relic of the past. It's boring and also has a boring story, at least in my opinion. The beginning is boring as all fuck and Star Wars itself is a boring universe. It probably picks up after getting off that horrible Sith starting planet but I can't be bothered to complete all the quests there to find out.
Mass Effect on the other hand has a modern story, well fleshed out and interesting characters, a decent combat system(2nd basically taught third person shooters a lesson if you ask me), a very interesting story with an extremely interesting backstory. All in all, Mass Effect is the better space opera here, at least in my opinion. I might be wrong as, like I've said before, I've never fully played KOTOR.