A little of both. Part of stability is leg tube diameter. For example, the RRS TQC-14 has diameters similar to a Gitzo 25xx series. It's generally thought that a tripod with 3 section legs will be more stable than one with 4 section legs. I'm not sure how true that is in practice, especially with a bit of weight on the hook. As I said, my TQC-14 travel tripod (4-section legs) does great for long exposures.
RRS legs are a bit more expensive, and IMO the fit/finish is just a little nicer (e.g., anodized aluminum parts vs. painted). But really, you can't go wrong either way.