I only just noticed they have the same base stat of 500. It's distributed slightly differently (but EVs could easily make up for it). Scyther has much worse type weaknesses though. Are there any good reasons to use Scyther instead of Scizor?
Scyther has STAB flying, which is better than STAB Steel (type coverage wise). Scyther has much better Speed Stat also, allowing for more possible sweeping. Also, you have immunity to Ground.
Scyther has the ability technician to power up weaker moves. Aerial ace has a power of 90 not including STAb, and infinite accuracy can be nice. He has higher speed than scizor, but still has swords dance for the nice power. Technician is the main reason people pick scyther over scizor sometimes. His type coverage is also different. Scyther has ground immunity and more type coverage with flying than steel.
defensive wise scizor has pretty good bulk 70/100/80 can definitely some take hit, with 9 weakness or immunity makes it a pretty good poke. while scizor is pretty slow, trick room can make it up and with a sword dance scizor can be really dangerous x-scissor and iron head for STAB and U turn when encounter fire type plus sword dance to boost attack to a dangerous level .