256 mb of RAM operating at 133mhz would be the faster option for most people. Extra RAM is only good up to a point. If you routinely exceed your available RAM by loading many applications or a couple of very demanding ones then adding more will help. However, I see some people under the delusion that adding stupid amounts of RAM like over a gigabyte to their system is going to make it faster. Usually, it just costs more money and does nothing because it is very rare for the system to demand that much RAM.
I say that 256 mb is the better option because, while it is not a huge amount by today’s standards, it should be sufficient unless you run huge, graphics oriented applications or massive applications like large databases. You will benefit from the extra clock speed if that is the case.
However, if your system often demands the extra 96 mb then you might be better off leaving it at the faster clock speed.
Of course, the best option is just add more 133 mhz RAM if you really need it.