Weber's website seems to imply that their grills have the same btu
output whether it's the NG or propane version.

When I bought a Weber NG grill I was very surprised to see that it
cooked much hotter than my older propane grill. They're not exactly
the same model (due to the age difference) but the temperature
difference was pretty big.

A factor to consider, in addition to fuel, is what the grates are made
out of. Cast iron grates take longer to heat up but they hold heat
longer. So you should remember that even though the air temp inside
the grill might be 500 or 600 degrees, the cooking grates might not be
that hot for a while.

Shaun Eli
www.BrainChampagne.com
Brain Champagne: Clever Comedy for Smart Minds (sm)