Swords Dance and Roost are both amazing options on Mega Scizor because they enhance its already great offensive and defensive presence. Get rid of Night Slash, as it is weak and doesn't hit anything super-effectively that threatens Mega Scizor from an offensive standpoint, so you can use both. Also, replace X-Scissor with Bug Bite, as it is stronger with the Technician boost. Or, you could replace it with U-Turn for a decently strong STAB which retains momentum.
If you HAD to choose one, however, it would really depend on the set. Offensive Mega Scizor definitely needs Swords Dance to break past its usual bulky checks, while defensive Mega Scizor absolutely requires Roost to maintain longevity and check the threats it's meant to.