On the muscle cars, no one really uses the power on the street and it really isn't practical to do so. Typically peak power is at continuous wide-open throttle close to the redline. Happens on a race track, not really on the street.
The air suspension allows load leveling and by its nature is progressive. If you are carrying loads in the bed or have a trailer with significant tongue weight, it is nice to have. If you just use it as passenger car or carrying a few 4x8s a few miles home from Home Depot, it won't be that significant. For off-road use the air suspension can gain extra ground clearance, and lowering for loading the bed is nice. People spend $10k to add that feature to 1-ton pickups. (
https://kelderman.com/)
The truck has a part-time 4wd, it is only actuated on slippery surfaces as it locks the front and rear differentials without a center differential. It also has a low range which is important on challenging surfaces when you need to go slow (walking speed or slower). Powering the front wheels is important when trying to steer out of snow, mud, or sand.
The best mileage is from the diesel...by far. And it has more than enough power; the first RAM pickups with the Cummins engine in the 1990s had 175hp. And there is a 33-gallon tank option. The diesel is an expensive option though.
3.21 vs. 3.92? The truck has an 8-speed transmission with a torque converter. All models have 0.67 top gear. On standard 32" tires it is the difference between 1700 and 2100 rpm at 75mph. If you plan on installing 33" tires or larger the 3.92 might be useful. The torque peak on the diesel is at 1600 rpm, on the Hemi at 3900, on the V6 at 4900. Between the two extremes—1st gear and 8th gear—the transmission takes over and the rear axle ratio is irrelevant. 7th gear with a 3.21 is roughly the same ratio as 8th gear with a 3.92.