The G3 is 3+ lbs and the SN2 is 5+ lbs so a lot more meat in the SN2. That said the only jaws listed for the SN2 and not listed for the G3 are the Powergrips and the 130mm. If you are doing normal depth bowls (3-4") or platters the G3 is probably fine. If you start going deeper or for hollow forms you probably want the SN2. IIRC the prowergrips are rated for up to 19" length (hollowform) or 27" diameter for bowls.
Insert and jaws are interchangeable. Sometime you can find the chuck (and key) without the 50mm jaws. I assume you have at least two sets of 50mm jaws already.
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