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Use Scizor in-game. Scyther's base speed is 105 while Scizor's is 65, but Scizor's base Attack is 130 to Scyther's 110. Scizor's base Def is 100 where Scyther's is 80. The other base stats are equal. Scyther is weak to Rock, Ice, Electric, Flying, Fire type attacks, while Scizor is weak only to Fire. Competitively, the speed advantage and STAB with Aerial Ace are basically the only reasons you may want to keep Scyther unevolved, since Scizor learns everything Scyther can (and more) and hits harder, with the exception of Aerial Ace.
I couldn't decide so I caught 2 Scythers and evolved 1 of them into a Scizor. Aside from stat changes they learn different moves at certain levels. So I suggest if you intend to evolve Scyther get it to one of those certain levels then evolve so you can have both moves. exp.(Wing Attack/Metal Claw, Air slash/IronHead)