Originally posted by LaurenOE Granted this entire setup is HEAVY, but light and stable don't go together.
Well, it's all a matter of how you carry the weight. It doesn't need to be a heavy tripod!
In the past, I've used an extremely heavy old Gitzo Studex 1500 (about 10lbs without a center column), but it wasn't nearly as vibration-resistant, or easy to carry, as my carbon-fiber GT3541. Granted, I'll hang a bag of rocks or sand (or a heavy camera backpack) from my center hook to "add weight" to my CF legs in windy conditions, but that's stability that can be added to almost any tripod. (Except really cheap "light tripods"... saw a friend's gimmicky "neotec" legs bend inward with a weight bag on them!)
Stan's advice above is on the money, though. Will reducing your load by ~2lbs make the difference between taking your big lens and gimbal, or not?
Personally, I've gone for two different "kits" when it comes to camera support, but both are very high quality and can handle far more than I'll ever put on them. One is small and light (Feisol 3441 & Markins Q3), and the other is (relatively) big and heavy (Gitzo GT3541, Wimberley Sidekick, and "a ball head to be named later"). The key is that neither of these is the cheapest nor the biggest, but strike a balance that works with my gear. I grab whichever fits the gear I'm going to put on top...
NB: unlike some other respondents, I don't have a Sigma 500mm f/4.5, only the (much lighter) 50-500 "BigmOS." Some of my pano-gear, however, can easily exceed the weight of a gimbal and 500/4.5, and has the unfortunate aspect of not being as balanced when you're moving it. Although I'm a pretty fit guy, I'll sometimes use a hand-cart to move the pano gear around, which might be an option for you. When seriously hiking from shot to shot, the Gitzo-ballhead-Sidekick-Bigma combo can be carried over one shoulder... if everything's screwed down and the legs are extended! (Padded Lenscoat leg wraps make it a bit more comfy!)