Paul. He actually would be a slight challenge and obeys the laws of Pokemon AIM FOR THE HORN.
I'd much rather travel with/battle/whatever with Paul.
Ash is lame.
srsly.
EDIT:
Plus, after watching episode 27, I have a preference for Paul's team. Torterra, Honchkrow, Electabuzz, Ursaing, Magmortar, and Weavile are much more rounded than Ash's measly bunch. I'm sick of his non-evo'd "power houses" and glad that Paul finally put that disgrace in his place.