Depends on your other team members, but since Pokemon Y is a fairly easy game, it's mostly personal preference. Heracross is a good physical attacker, but Lucario is faster and can be a mixed attacker but has less bulk than Heracross. I prefer Heracross though, love its design and name and using it in SoulSilver. I think Lucario is cool but has been pushed down our throats too much since its debut and has been getting too much special attention.
I think this thread should go on the sixth gen section since this isn't competitive related.