To me the Rugers and Colt replicas, whether they be cartridge-firing or percussion, are as different as night and day. I love them all and each has its place. The Ruger is a thoroughly modern design, based on the historical Colt SAA. They are well made, are usually plenty accurate and will last practically forever. The trade-off is that they do not feel like a more traditional single action. If you love the traditional guns, they can leave you a little wanting. Especially the original large frame Vaquero. It is nowhere near as svelte and slick-handling as the SAA. The New Vaquero, with its smaller frame, is much closer to the Colt SAA.
The Colt and its replicas are almost exactly as they were over 100yrs ago. Maybe not quite as nice in terms of fit and finish because of the amount of handwork present in the originals but the overall appeal is there. They take you back in time like no other and can be every bit as accurate as a modern gun, if not moreso. The crude, original-style sights usually being the weak point. The trade-off here is the very old design, dating back to the 1836 Paterson. Now I believe that much more has been made in print about the fragility of the Colt designs than exists in reality but the possibility for mechanical breakdown exists. As with any other man-made contraption. A professional tuneup can prevent much of this from ever happening. Nothing feels like a tuned Colt!
These days most of the Colt replicas are very good and have been getting better in recent years, particularly the SAA replicas. My advice would be to buy the best you can afford, expecially in the percussion pistols. Avoid the cheap guns and go for the newest of production guns. I also believe the cartridge conversions to be typically of better quality and made from better steel (or at least better heat treatment) than their percussion counterparts, being more heavily regulated by the ATF. Of all those I've laid hands on, the Cimarrons seem to be the best.