i, personally, would go with higher Mhz, but remember, the faster memory can always run at lower FSB with tighter timings, and memory CAN be overclocked (i got my pc2700 running at pc3200 specs)
so when it comes down to the choice of mem, i would go with the cheapest GOOD stuff you can find (for instance, if the 3500 there is cheaper, go with that, since they both prob use close to the same chip, if not the exact same)
others can prob help you with this more than i can tho, but for P4s, i believe higher FSB is always better than lower timings, since the FSB would benifit the entire system
EDIT: just like you might not get your sys stable at the higher FSB, so why pay for something you might not even be able to get?