This could go on forever. My suggestion is to spend a few minutes on Google and search for "DDR vs. RDRAM." You'll see tons of articles, reviews, and benchmarks comparing the two.

Originally high cost was a major turn-off for people looking at RDRAM, but lately it has dropped to be fairly competitive with DDR and the like (although still slightly more expensive). What you pick really depends on what you currently have, and what you're willing to spend. If your motherboard already supports DDR, RDRAM, or SDR and you don't want to spend the money to replace it, then you'll be stuck with what it supports. There are a few boards that support more than one type of RAM, but they tend to lack in other areas. So, it's best to choose a RAM type and stick to a good board that supports that type.

Anyway, a lot of it is personal preference. I personally stick to DDR, but that's just me....