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Mostly personal preference and situation dependent. The further out your hold your rifle the faster you can swing or 'steer' the rifle. The mag well hold is nice for extended firing/exposure.
I use both. A VFG that is pretty far out, like TDC said, helps me with snap shots. When I'm prone and a 30rd mag is resting on the ground, then I use the magwell as a grip.
I think we all agree, VFG out far for 'normal' use and a mag well if you want to stick your elbows in your chest for shooting at distance on your legs unsupported.