Definitely Paul for me - imo he's exactly what the show needed, and is probably the character I'm interested in most this season. He's a total prick to Ash and his pokemon, and you can't help but wonder why he treats the pokemon the way he does, so you're always waiting for him to come back so you can find out more about him. He shows up a lot more than Gary did, and does a lot of other things besides battling and teasing Ash (Roark's Gym, Cynthia ep, recent Tag-Battle eps), which I think is really great. I can't wait for him to show up again, and to see what he's like by the end of DP.
Don't get me wrong, I still like Gary a lot (well, after his comeback at the end of AG, I didn't really care for him too much while he was actually on the show), but when I compare him to Paul, he isn't all that interesting to me, besides his whole Pokemon-Researcher-thing and the backstory with Ash. So yeah - Paul wins for me.