I've used the West systems and the Pro Set (both from same parent company) and there is a marked difference between the two. System 3 also seems to be pretty good and pretty consistent. The West systems seemed to just not harden as much as the others and may have a lower temperature tolerance, I don't know - I think I remember something to that effect. Anyway - for general purpose use, the West system isn't bad and easily available, but for something more critical, I'd probably spring for the extra.
Having said all the above - I want to try a Vinyl Esther resin for building a mold so I can use typical gel coat tooling, etc. and not cost as much, if nothing just for kicks and the education of it all. It seems to have promise.