You can't think too carefully before you buy a tripod. There's a definite trade off between weight, portability and rigidity. Whether we like it or not lightweight doesn't usually mean rigid. If you go too light the slightest breeze will set it moving, and remember you may be mounting some pretty heavy camera/lens combos on it.
Make sure you get good rigid legs and as portable as possible with closed length being a big consideration - you can always change the head. I have a Manfrotto tripod, the legs are ace even after 20 years, but it has a large 3 way head which is heavy (originally bought to support medium format) so at some stage I need a lighter head, so now you've started me off again.
