If you are talking stock cars, 6.6 vs. 4.9T, chances are, it will be the stock 6.6
However, that's not always the case.
The stock 4.9T was a heavy car, the heaviest Trans Am yet for the second gens. But, the 301Ts got the 3:08 rear end gears.
However, bad points.
The turbo has the world's smallest oil cooling lines! They heat up fast. Very fast! Let them cool down, before shutting them down.
Lot's of turbo lag, at least, compared to the 3.8T!
Computer crap. I heard that you cannot find new, so junk yarding is a must.
I had my 301 Turbo rebuilt (just the turbo) 3 times already! And the car only has around 80,000 miles! I use to get it done for $150! Not now, much, much more!
Good points.
As mentioned, the 3:08 rear!
Most come with the WS-6, which with that setup, you can keep up, if not, pass others in the twisties!
Sharp looking cars!
Now, since you are talking about the 6.6 auto, that means just the Olds 403! The only Pontiac 6.6, was the W-72 in 1979, which was the 400 4 speed.
I have one of these too, and they are a blast to drive, even if they have less horsepower (185 vs. 210).
Bad points.
Most had the 2;43s, some had 2:73s.
Alot of them came without the WS-6, so no posi, big sway bars, etc.
Good points.
Could smoke any car from the same year (can't do that with the 301 Turbo, as the Vettes had the 350 still!)
Easy to find parts, compared to the 301T. At least, GM parts.
No turbo to rebuild all the time.
Now, not alot of mods to do with the 301T. I had NOS installed. Scared lots of people, since it could do nasty burnouts from a dead stop! No more turbo lag, however, even when rolling at slow speed, would fry the tires! Can't swap other engine parts (with other Pontiac engines). No head swaps, no bracket swaps, since the AC units changed. No intake swaps.
But, the 403, you can swap lots of things.
I did a 3:08 rear change (big difference-now do burnouts all day long!).
True duals, pick up power. Can't do that with a 301T.
Change heads with other Olds engines. The popular is the 350 head swap.
Heck, you can even do header swaps.
While the 403 was used for 3 years in the Trans Am, and the 301T only two years, the Olds 403 was used alot by Olds. So, lots of parts!
Heck, even the aftermarket has alumi heads for this engine! Not for the 301T.
I own all three of the cars (301T, 403 Olds, and the 3.8T), and I love them all!
But, I would get another 403 before I got another 301T, unless it was a collector's version (NASCAR Edition, Indy Edition).
But, the final choice is yours. Do what you can afford. You will have fun with either.
George