I've had about 6 different rear spring pairs in my 4Runner, with many more sub-combos of packers and air bags. I have both high and variable weight in the back, since I have a 4XI swingout bumper and tow a light car trailer from time to time.

I ended up with the Superflex coils, simply because they have the highest "final" rate of any of the heavily progressive coils and prevent big rake changes with variable weight. This is a huge selling point, especially when they still ride very nicely sans trailer.

Unfortunately, air bags aren't quite the silver bullet they'd seem to be for variable weight, since you can't safely run them at 0psi. I always found them a bit harsh unladen, even inside soft coil springs.