What motor is more reliable between the 3800 and 3800 supercharged?
Same motor built on the same line by the same people. Either is only as reliable as the maintenance that is done on them. A 3800 was recovered from a wrecked car in taxi service with over 300,000 miles on it. The oil was changed every 3000 miles. The crank showed so little wear it could have been put back on the line with the new cranks and you couldn't have told the difference. Regular maintenance counts!
What is going to get better mpg?
Depends on the driver. On paper the supercharged gets better mileage by virtue of the 2.93 gears verses the 3.29 gears in the GT. Lower revs equals less fuel. I've got one of each and the GTP gets better milage IF you can keep your foot out of it.
Anything to take in consideration?
The earlier cars (1997-2003) had motors that were "hot tested" before they left the plant and were assembled by people with decades of experience who could do it in their sleep and were proud of the multiple awards GMPT Flint North won for the 3800's quality. Towards the end they got rid of hot test to eliminate jobs and assembly was peformed by temps as the more experienced workers left.
If it were me I would snag a GTP from 1998-2002. Oh wait, I did.