I've seen some demonstrations of people hanging from/standing on their tripods, so I've concluded that strength is not the most important criterion. Stiffness is what keeps the tripod from shaking/vibrating.
While the two are related, they are not the same. Stiffness is affected by the diameter and modulus of the tripod legs, as well as the stiffness of the head. Stiffness is very hard to measure, so manufacturers give us a guide by quoting weight capacities.
Having bought four tripods and disposing of three of them, I well know the difficulty in finding the "right" one (many users have several).
The advice given above to try one out is sound IMO. Mount the camera with lens on the tripod and give it a bump to make it vibrate, then compare another tripod using the same technique. All tripods will vibrate to some extent, but some will vibrate less.
I personally use a Gitzo Explorer (which someone on another site called a difficult and awkward tripod to use). The legs can't be pulled out, they must be unlocked first, then when in position, locked again. This is awkward, but the benefit is that the joint between the head and the leg is stiff in both directions (in and out). A tripod on which the legs will pull out is stiff in one direction of leg travel only (against the click stops) - pushing the legs back in offers little resistance, and no stiffness in that direction.
Hoping that this makes sense.
Click on the image, and the locking levers can be seen. The twist locks (g-lock) are very nice.http://www.gitzo.us/series-2-carbon-6x-explorer-tripod-4-section-g-lock