I'd choose Lucario for three reasons:
- Liepard's got horrible stats compared to Lucario's.
- Lucario's stats increase even more with it's Mega Evolution Stone!
- Lucario gets a heap more resistances as opposed to Liepard. Of course, in Gen V Liepard only gets two weakensses as opposed to Lucario's three, but then Fairy came into the game and ruined everything for Liepard. :3
So, sorry Liepard. I do like cats though. :3
Hope I helped. :)
Source: Serebii's pages for the Pokemon. :3