I personally prefer Heracross.
It depends very much on what kind of Pokemon you like more.
Heracross is offensive and Scizor is defensive. Scizor's Attack is high, but not enough to classifies it as a sweeper and the only fact that turns it into a defensive Pokemon is the metal steel type, and not the defensive base stats.
They're both 4x weak to a certain type of move. Heracross for flying and Scizor for fire. Once that fire is more common to find out and Defense is something that you can raise up with Bulk Up, I think that Heracross' weakness is easier to get along the battles than Scizor's.
Heracross also got a rare move named Megahorn which is 120 power and 85 accurate. Scizor doesn't have any powerful move like that. If you want to turn Scizor into a sweeper you'll need to use some TMs on it.