I wish there was an option for both, but I chose Paul in the end.
I like both Paul and Gary, before Paul, Gary was my favorite, he just had this thing about him that I loved and even though I preferred Jerk Gary over Nice Gary, it doesn't matter, because I just can't see myself not liking him, plus I've always preferred Gary over Ash anyway. Plus he has an awesome VA like Jim Zoppi to voice him. I'd die alittle inside if they'd chage Gary's voice, it wouldn't feel the same.
Paul. He's in my opinion the best rival for Ash, its a change from all the nice guys we see. Sure, Paul's harsh on his Pokemon, but that's the way he trains and so what, I mean its not like he's abusing his Pokemon... physically anyway, Chimchar was a bit mentally scared I think, but Ash and his magical Power of Pokemon Friendship(TM) cleared that up, so the monkey will be alright. Paul's not necessarily a bad trainer... I mean none of his Pokemon hate him, he's just.... different.... his Training methods are different I would say and I mean this isn't anything new, all Paul is, is just the animated version of Silver from G/S/C
And in G/S/C did most of you really have a problem with "Silver?" I didn't, I mean hey not every trainer is going to be all nice and sweet and kind to their Pokemon, I agree with those who like Paul and said his character is realistic, its true... I think out of all the characters on that show, he's probably the most realistic one, he's not just another cardboard cut out of Ash. He's not like Tyson, or Morrison, Ritchie or Gary, he's different, he's snide, prefers strong Pokemon over weak and scolds his Pokemon if they lose and if he doesn't want them, he'll get rid of them, rather than keeping them and abusing them further.
You'd dislike Paul a whole lot more if he did that am I right? So, who cares if he releases weak Pokemon and scolds the ones he deems strong enough to battle by his side after they've lost, he might not be the perfect trainer or the kindest one, but I like him anyway.
@Neiteio: I don't think Paul's anywhere near Time-Skip Sasuke's level of douchebagery. Paul doesn't angst, he's not out for revenge, I think he cares deeply for his brother, even if he doesn't show it, he doesn't hate everyone, he respects his elders has mild respect for Brock, though due to association with Ash, he shows mild distaste as well, all the while feeling indefference to Dawn, plus he doesn't hate Ash. I mean, Paul barely even recognizes their rivalry and Ash as a trainer and ignores his speeches about loving his Pokemon, the only thing he's remotely interested in is Ash's Pikachu (Because everyone loves that stinking yellow rat) *insert eye roll* Other than slight interest in Pikachu, he could probably care less about Ash and care more about defeating the Pokemon League and gaining strong Pokemon.