Looks like the 3 tons are back ordered online and not available for me locally. Must have had a run on them. I could buy up, but my largest vehicle is a 99 Chevy Silverado 1500. I'm assuming much larger vehicles require the 6 ton.
The Northern Tool Strongways are made by Torin. You might check ebay for them or the Torins, as Northern Tool has an ebay storefront and I once got a good deal on the 6 ton Strongway on ebay, around $32 delivered after an $8 off ebay coupon.
It is not really a matter of needing more tonnage. Rather I would get the tallest stands that are still short enough at their lowest height to fit under your vehicle at the desired locations once jacked up.
This also means, all else equal, that they'll have a higher max lift height which can be VERY useful for working under the vehicle. A few more inches clearance can mean a lot in what kind of contortions you have to do or amount of leverage you can get without as much fuss, and the larger base footprint means more stability.
I don't know about your '99 Chevy, but some newer trucks and SUVs can benefit from taller than the average 3 ton stands... but along with that, if you don't have a high lift jack, you may not get the full range out of a 6 ton set, unless you put an extension on the saddle to get more lift.
I have both 3 and 6 ton and only get out the 3 if the 6 won't fit. So in other words, get both... or jack up your vehicles and measure what'll fit where you want placement.
