You're completely confusing these 2 things.
1:1 is the actuation ratio of the SRAM stuff. 1 mm of cable pull on the shifter moves the derailler 1mm. It's the 1:2 on the Shimano stuff. In theory, this gives the SRAM stuff a greater tolerance for being out of whack and still working right.
If you take the cable off of a normal derailler, the spring will pull it towards the smallest cog. By pulling on the cable you move it to bigger cogs on the cassette or lower gears. On a Rapid Rise, it works the other way. The springs pull it towards the big cogs and the cable pulls it towards the little ones (or higher gearing). For the life of my I can't figure out why Rapid Rise exists, but if someone likes it, then good on 'em.
If you were going to use a SRAM rear derailler, then you'd also have to change the shifter because of the different actuation ratios. For that reason, it's going to be out of your budget (except on a sweet Ebay deal, maybe). XT is really, really good stuff. The only difference is that it'll be a pinch heavier than XTR. My advice is to stick with an XT rear derailler.