...They're used for totally different things, so I don't know if you could establish a comparison between the two.
Scizor is used to tank in the Active spot with a few Special Metals and possibly Expert Belt, letting it hit 90 or so a turn and basically transforming itself into a mini-Donphan Prime, and that also gives DCE grief.
Luxray and Garchomp usually don't hit the Active spot; they're used to sniping around everything rather than just a head-on assault.
I personally think Luxchomp is better due to the disruption factor, and in the current meta, disruption usually is more an effective deterrant than walling, although Scizor Prime is definitely not without charm.