firstly: edit your post, dont double post
secondly: i would reccomend scizor but i also like crawdaunt more as of our little bond in the third gen. would you give us movesets?
if no then i would say go with crawdaunt.
Although Giga Impact may seem strong, you must take into account the loss of a second turn. Being able to use the same attack two turns in a row will often be much more powerful than sacrificing a second turn to Giga Impact. Your Scizor, for example, would be much better off using X-Scissor twice than using Giga Impact.
ok lets see, Crawdaunt knows Knock Off, Brick Break, Night Slash and Crabhammer, though Knock Off is what I'm going to replace for Giga Impact, while Siczor knows Metal Claw, Razor Wind, Slash and Agility.
Don't put that on anything other then Slaking, because you'd lose the extra turn anyways. If you had to pick, pick the weaker of the two, because it'd make it stronger... I think.
Scizor's moveset is pretty bad, but it is likely that you can do more damage in two turns with Slash than you could with Giga Impact, and Slash isn't even that great.
Don't use Giga Impact, Hyper Beam, Blast Burn, Hydro Cannon, Frenzy Plant, or Rock Wrecker. You'll waste a turn recharging, and you also can't switch your Pokemon. (also a reason why Slaking is always better with Return over Giga Impact.)