IIRC the XP1600+ runs at 1400Mhz. Assuming (yes, I know what happens when you ass-u-me anything) that, you would need to run at a multiplier of 10.5 and FSB of 133 (10.5 x 133 = 1396.5) to achieve stock speed. Being under the assumption that your CPU is multiplier locked at 10.5 and you are running at 146FSB (10.5 x 146 = 1533), I'd say that WCPUID (10.5 x 146.76 = 1540.98) would be closer than MBM5 (10.5 x 146.95 = 1543). Of course this is based on some rough calculation and guess work and I could be wrong.
Edit: After doing a few more calculations, if you were running at a 147 FSB (10.5 x 147 = 1543.5) then I would tend to believe that MBM5 at 1543 would definitly be the closer of the two.
Now that I've confused myself thoroughly, I'll let you ponder on my conclusions for a while.