Are they the same arms and the [60gm?] weight difference comes from different chainring materials? If so can I put XT chainrings onto an LX crank? Or are the arms themselves different?
Reason I'm asking is that my full suss is not going to be used for quite a while and my regular ride 456 currently has LX on it, so I was thinking about swopping them over.

Parts are interchangable, and rings from either will fit arms from either.

The arms are quite different on both though, not just the rings using different materials (iirc only the inner ring on LX is steel, the others are ally too). LX had an ally axle too, which means they're not up to as much abuse as the XT with their steel axle.