Strictly speaking, 256MB of SDR would probably perform better, but it would depend on what you are doing.
DDR has a larger overhead than SDR, so if your bottleneck is somewhere else, the extra latency penalty that you get for using DDR will produce a slight performance hit.
eg, compare a DDR GeForce 2 MX with an SDR GeForce 2 MX.
Secondly, (I assume you are talking about system RAM here), if whatever you are running is not going to fit inside 128MB of RAM, then you are going to get hard drive thrashing, which is going to be far slower than what would have fit inside all of 256MB of RAM.
If you are never going to need more than 128MB of RAM, ie everything fits in RAM, then the DDR will be faster.