The first thing you need to do is figure the answer to a few questions:
1) What is the maximum weight you will be putting on the tripod? This includes body, maybe a grip, battery, lens, hood, flash, etc etc etc.
2) Are there any special requirements such as a specific minimum distance from the ground? Ability to hang out an arm? Need to add weight bags in high wind?
3) How far will the tripod be carried and what will it be carried with?
Here is an example of how I came to my legs and head combo for my DSLR:
1) 5lbs 12oz (Nikon D7000, grip, battery, Nikon 80-200 2.8D, hood, cap, quick release plate)
2) Will be used for macro so close to the ground and "special" situations are a a must.
3) Will be carried on hikes with lots of other equipment to the lighter the better.
This led me to Manfrotto 190CXPRO3 legs as they will support up to 11 lbs (twice my max camera weight, or my max camera weight and a 5lb weight to help stop the wind from knocking it over), these legs also have a center column that will come out and lock at 90 degrees to allow for all kinds of weird angles and positions and only weighed 2.8lbs, I coupled this with a Manfrotto 460MG magnesium RC2 head that supports over six and a half pounds while weighing only 15.5 ounces. The RC2 compatible head also allows me to use the Black Rapid RC2 setup so I can use my camera on my strap or on my tripod without having to worry about taking things loose.
Here is an example of how I came to my legs and head combo for my large format camera:
1) 10lbs (4x5 monorail fully ready to shoot)
2) Extreme stability required.
3) Will never be carried by itself, always transported on a dolly with the camera.
Here I chose a Slik Pro 700DX Tripod with 3-Way Pan/Tilt Head as it supports up to 15lbs and my 4x5 monorail weighs just under 10lbs. It will never be carried far so weight was not a concern. The head works fine because you can release the pan and tilt controls just a tiny little bit which works great on large cameras where the sudden release of tension might completely drop the camera. One of my main reasons was I actually got to test this unit in the store before I bought it and in it's size range it was by far the most stable unit with a load on it in the store, and then when you factor in the price, it was a steal.
Your setup will surely be different but that should give you an idea of how to approach it.
Allan