Bullet Punch = 40 Base × 1.5 Technician × 1.5 STAB = 90 Metal Claw = 50 Base × 1.5 Technician × 1.5 STAB = 112.5
22.5 difference. The gap widens when you mix in additional modifiers. But it's negligible when you consider Bullet Punch's priority and the fact that it has 100% accuracy vs. Metal Claw's 95%. And it's still very powerful. Swords Dance Bullet Punch is just plain vicious.
If you need more power, consider keeping Bullet Punch and using something else. Bug Bite is a good one. (60 base damage, Bug type! With technician and STAB it gets to 135 -- even better than what you'd get with Metal Claw)
Bullet Punch is the single reason Scizor is so damn popular. If it wasnt for that move, he'd only show up on teams with people trying to be "original". Having a STAB'd priority move with a Technician boost is amazing.
Scizor was BL until HGSS and was really one of the more usable BL's. Even without a reliable way to threaten things, STAB U-turn with awesome stats is just sexy. Plus, with his decent support movepool, he had a niche to fill here and there.
BTW, don't be mislead, SD Bullet Punch is good, but far from amazing. Scizor's main draw is his ability to scout with U-turn (which is complemented by Bullet Punch), not his ability to sweep. Most good Scizor have a Choice Band.
If you are currently in an argument with me, consider the following: http://www.qwantz.com/comics/comic2-1673.png