To answer the original question:
In theory the LSW takes only magazines, usually 30 rounders, while the US SAW takes either 30 round mags or 200 round belt.
In practice I heard the same thing that the SAW doesn't like magazine fed operation.
One advantage of the magazine is the rounds don't rattle like in the belt fed box/hopper.
Both LSW and SAW are intended for single soldier operation, unlike crew-served guns like the heavier GPMGs.
Jeff White: the RPD is a Soviet design for a 7.62x39 squad automatic (belt fed by the way). It really isn't in the GPMG class. The Russky equivalent of the GPMG is the PK series of 7.62x54 machine guns.
Other than that I agree with everything you said!
I read an interview of Peter G. Kokalis after he'd come back from Bosnia or thereabouts training Croat machine gunners. He commented that the choice in MGs is either:
1) Fires a powerful round that punches through cover and works out to a decent range but is too heavy to carry much ammo without help
2) Something easy to carry that doesn't have as much reach or power in the cartridge.
What's the best answer? I dunno. Murphy will see to it you'll bring the wrong one.