I currently use both programs (Mac versions). Carrara has most everything that RDS 5.5 has and was indeed intended to replace RDS 5.5. There will be no further upgrades to RDS. Carrara has a completely differet interface - very similar to Poser & Bryce, if you are familiar with those programs. It is much faster than RDS in most areas particularly in on-screen rendering and in the mesh form modeler. The big advantage to me of RDS, besides the fact I've used it for years and am more familiar with it, is it is more stable (as long as the appropriate patches for your particular system have been applied) I can work in the program all day without a crash or freeze. Memory requirements are less for RDS for it to function well, but ALL 3D needs lots of memory to get the most out of it. Carrara is buggy. It seems to be on an almost individual basis as to how buggy. The Mac version seems to have more problems. I believe most of those problems are related to the program's "access memory routines", since that is when all of my freezes and crashes take place. However, by changing the way I work, I've been able to run the program in a semi-stable fashion for longer periods and get more done. Now the bad news: The future of RDS is Carrara and the future of Carrara is in limbo. There are encouraging reports that Adobe or the original engineers or both will continue to develop Carrara, but they are all playing the "Can't talk about it" game. If you want a program as a workhorse that you can crank out models and scenes in a stable environment and aren't as concerned about render speeds, having a new version come out, or the latest bells 'n whistles, then RDS is a great program (it has quite a few bells 'n whistles itself). If you can tolerate the occasional freeze or crash (you may not have any), want a really cool and easy to use interface, need a better mesh form modeler, faster rendering and a very powerful shader tree system, plus the "possibility" of an upgrade, then go with Carrara; it is RDS on steroids and has a tremendous amount of potential. Just my two cents on the subject.