Well that depends on serveral things
RDRAM has a bigger latency than DDR which is not good and only works in pairs.
RDRAM is around the same price (and even cheaper since the last ddr price increase) as DDR although a 512MB stick of RDRAM is very expensive indeed. RDRAM is better for office use e.g. for graphics design and is faster than DDR in Quake 3 by 7%.
as far as what will be used in the future on Pentium 4, who knows, but faster RDRAM is coming out soon 1066Mhz and i'm sure DDR will get faster too.
Although my pick on which ram depends on the speed of the P4.
2.0a-2.2 Northy, I'd go RDRam
1.6a-1.8a - I'd go DDR
RDRam I heard gets stuck at 150+FSB, and the 1.6a and 1.8a overclock very well, even a possible chance of 150+ and I wouldn't like ram holding me back from that, and the only possible way of reversing it is a new mobo and ram.
RDRam is very power with big things like those 2gigers, the 2gigs won't get as high in FSB as the lower chips, this is where RDRam kicks in and does it stuff. Without the high FSB, you can use aggressive memory settings which will make the RIMMS fast and beat out DDR ..