I believe Scizor is better than Scyther.
That 20 more Atk in its BASE stat actually does make a difference, a HUGE difference actually. That is like saying Blaziken's base 80 Speed isn't much different from Charizard's base 100 :/
Scizor gains a poison immunity as well as a few more resistances. Scizor is more durable. It also gains Iron Defense, which can be used to BP some defense to something like Gardevoir, Regice, or something who is higher SDef than Def. It can also learn Agility, so it can sweep just as well as Scyther(it won't need Swords Dance, its Atk is already really high).
However, they both aren't very good, due to their limited movepools. They get much better in D/P though, moveset-wise.