Scizor > Bisharp
- STAB Iron Head
- Access to reliable recovery - Roost
- STAB Technician boosted Bullet Punch
- Resists Fairy-moves
- Lesser weaknesses
- Slightly lesser Speed -- Fixed by Priority
- Better Speed
- More resistances
- STAB Sucker Punch
- Normal damage from Fairy type moves
- Sucker Punch isn't exactly the best move for a (faster) fairy
- Weak to Mach Punch
- 2 more weaknesses than Scizor
All that summed up, I think Scizor wins out. Added bonus if you can mega-evolve, but works wonder with a Choice Band too. Or try Life Orb with Roost support.
Finally, though, the choice is yours. You can try out both to see how you play with either of them. Chances are, you'll get to see how the synergy between your Pokemon will work out, and you can probably come up with a new strategy!