Either works well, but I'd go for Tyranitar. With the moveset in this answer, it can take down 2/3 of Leon's team super effectively.
Tyranitar @ anything
Ability: Sand Stream
- Stone Edge
- Fire Punch / Thunder Punch / Ice Punch
It also has solid defenses and an excellent Attack stat, even if it is hindered a bit by its slowness. Leon is somewhat easy, so you shouldn't have much trouble beating him, no matter which one you choose.
Hope I helped!