This is another interesting question like my previous about distros.
I'm currently in the process of updating my old hardware but I'm stuck on this "simple" question

Should I buy an AMD ATHLON XP 3200+/3000+ or a Pentium IV 3.0 Ghz HT Prescott? I know there are better alternatives like AMD 64 bit but say my budget is limited to the former.
ATHLON is cheaper and according to most of the benchmarks I've seen it outperforms Pentium. But when I go to the stores the salesmen say that Pentium is faster. I don't believe them a 100% but I'm not that expert.
The main use is for running sims and mostly calculus.
What are the expert suggestions?

Thanks in advance.

Cacarulo