If these are both "non-lettered" versions (no SE, LE, etc.), the X300 has a faster core clock by 75MHz, probably more efficient, and generates less heat. There may be other process improvements (RV370 vs. RV350), but otherwise the cards have identical "paper" specifications. Both can be found with 64MB and 128MB. 256MB is also possible but watch out; Many are SE versions with the memory interface chopped in half to 64-bit.

Here is an overview of each (non-lettered):
https://www.techpowerup.com/gpu-specs/radeon-x300.c511

https://www.techpowerup.com/gpu-specs/radeon-9550.c33

There are also hypermemory versions of the x300 around, which are of course to be avoided.

so yes the ati radeon x300 128mb is litte bit faster than ati radeon 9550 64mb