I use my 6 ton stands where they will fit, and where I can get use out of their higher extension (~24") compared to my 3 tons stands (~17"). Granted I don't extend either up to their last height notch so there is more purchase on the shaft for stability.
I don't have an especially high lift jack so sometimes I can put a block of wood (or I could break down and buy a proper extension for it) on its saddle to get more lift to use the 6 ton, but this is not always practical nor needed.
Cars doing wheel work, 3 ton is enough. Trucks and SUVs, depends on the wheel/tire size. Working under any vehicle, I really like having a few more inches of clearance using the 6 ton. Often I could put the vehicle up on ramps but prefer to use the jack stands.
If I had to own only one pair, it would be 6 ton for trucks and SUVs (real ones not slightly higher riding unibody station wagons), 3 ton for cars.
I also have some stray little 2 ton that I sometimes use on a riding mower and not much else.