It all has to do with the frontside bus. Say if you have a 200Mhz (400Mhz effective) FSB and multiply that by 11 then you have a processor clock of 2200Mhz or 2.2GHz (which is Athlon XP 3200+ speeds by the way). Since most newer Athlons have an overclocking ceiling of somewhere around 2.3GHz most people will lower the multiplier so they can raise the bus speed more. Raising the bus speed means that the processor can transfer more information per second.
1. 200FSB x 10 multiplier = 2000Mhz
2. 133FSB x 15 multiplier = 1995Mhz
Even though the clock speeds arn't that far apart by the fact that the 1st Option has a faster front side bus (grand total of 3.2GB/s of bandwidth compared to 2.1GB/s) it would probably have greater performance.
Hope this helps!
-Por