The TQ will give more mpg because it mixes the fuel better in the very small triple venturi primaries, it will also be more responsive but a 500 inch BB with any 950-1000 carb should be very responsive. The one thing you can do with the TQ is bolt basically the exact same carb on a stock 273 and still have a very responsive motor, 6 pack, not so much.

I think air flow "up top" would be the same, but if I was forced to pick one I would guess the TQ with it's sewer pipe sized secondaries would flow more. Also it comes back to the manifold selection, a big single plane can bolt right on under the TQ and have a big plenum feeding all 8 cylinders from all 4bbls, the 6pack, well I don't know of any single plane for them, probably some odd balls out there but not many.