I've created and published archetypes, alternate classes, monsters, feats, templates and spells, and strictly speaking, I never cared to until I started using Pathfinder. Because all of PF is OGL, while only some of 3x is OGL, I'd stay far away from 3x, and just do Pathfinder, at least from a 3PP point of view. Having created all that I have so far, I'd say PF is easy enough, even logical - of course never having done so for previous editions, I can't say if it is more difficult than doing so for 3x nor any other edition. Again, I find designing PF content pretty easy and intuitive, once you get a grip on the rules.
Consider that I'm really just a freelance cartographer, but have managed to get a PF 3PP, Rite Publishing to let me publish my own setting as an imprint under Rite Publishing, while still remaining my IP - the
Kaidan setting of Japanese horror (PFRPG). Right now I'm working on writing a series of map tutorials guide books following a successful Kickstarter last year. When I have the time I will create some more content for Kaidan, perhaps looking to develop a full AP.
Don't let others scare you away from using PF for your setting, it may be too tough for them, again, I find it fairly easy and intuitive, although it is certainly lots of detail work to get everything right.