Well, if you're benchmarking the cpu only, yeah.. 7x138 will yield better results. But the cpu isn't the system. A higher FSB with a lower multiplier for a given resultant speed increases the entire system efficiency as data moves to/from the bus and memory at higher speeds, rather than just having a faster CPU only. For the case of 1GHz CPUs... 7.5x133 or even 7x143 is better than 10x100. As for your case ( different end-speeds ) however, I don't know if a 12MHz FSB increase would necessarily justify a 66MHz cpu speed decrease as far as overall system performance.
I hope this helps a bit,
K-Lined4Lyf