There is literally no performance difference as far as the shot is concerned (I can't believe we're still talking about that...) but the fit and finish of the Carter gun is generally superior, as each gun is made by hand. They're also Cocker threaded, and don't rely on detent rings (an arguable downside) to shoot right.
Nowadays, the leading reason is probably just the customization.
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Can't we all just play Paintball?