Mosel is a chip manufacturer - They make the memory modules themselves that are then attached to the PCB. Equally clocked, there will be no performance difference whatsoever between Mosel & any other brand. PC133 CAS2 is PC133 CAS2. However...
The real difference shows when you start cranking the FSB. Mosel is rated at PC143 CAS2, & is known to run 155 or higher at CAS2, something that until recently very few modules could do.
In short it's damned good memory, but not worth the premium if you don't plan on cranking the FSB well above 133.
As for the 256 vs. 128 debate, it depends on your preference. I don't believe there's any real performance difference between the two, except possibly with interleaving... But I don't know for sure on that, I haven't toyed with any AMD boards yet (I'll do my first tonight, my HS finally arrived!). If you want to pack the most RAM possible into your board, use 256 or 512 DIMMS (if your board supports them). If you want flexibility (eg remove 128 later to put in a different system & replace it with one 256), get two 128's. With RAM being so cheap, though, stock up on 256's. That way you get the best of both worlds.
Viper GTS