Story, gaming experience, all that stuff is very subjective. As is said in every opinion thread, we don't know your likes and dislikes. One's a WRPG, the other's a JRPG. If you're interested in both, I'd assume you know the difference. I'd also assume you know which genre you prefer over the other. That said...
ME2 has had a steady stream of DLC since release. New missions, new characters, new weapons...most of which will be free if you own the game. FF13, on the other hand, has no DLC, and I haven't heard a single thing to indicate that it ever will. So, ignoring all personal preference, FF13 would make more sense as the rental and ME2 as the buy.