Bernard Miller , Jul 23, 2011; 05:05 a.m.
Well, Angel, there is loads and loads of information on the internet about RB67s. If you do a search here at photo.net, you will find practically everything you'd want to know--including plenty of user experiences. You just have to look a little harder!
I own one, so I'll give you my experience: it's indeed a superb camera, the lenses (most of them at least) are extremely good performers, especially if you go with the newer versions (KL-L are the latest, C are the version before that--still very fine lenses) and you'll get amazing images. The camera is vastly larger and heavier than your Seagull, and has a huge mirror that flips up, as it's an SLR. For some things, a tripod would be almost essential with it.
The 50 millimetre lens available for it will give you a much wider angle of view than your current 75.
If you don't like the idea of lugging around an enormous camera that can be a bit of a pain to use outside a studio and you like the feel of your TLR, why not look at a Mamiya C220 or C330 model with a 55 mm lens? It won't be quite as wide as the 50 on the RB67 would be, but you'd still get excellent quality and a lot more portable and hand-holdable camera.
Best of luck!