Both are great boards, but like wild_andy_c said, the +IV is better. It's a later design that improved the +III in many areas in addition to the HPT366 controller. I've been running the +IV since April and it's the most stable board you can get. And it overclocks great. Get one you won't be dissapointed. With the % core changes in the BIOS it's ideal for overclocking with a slotket/FC-PGA setup. Whatever you set as the base core on the slotket, you can adjust upward by 2.5%, 5%, 7.5%, and 10% in the BIOS. So you never run out of core voltage settings. Plus it works great with SoftFSB.
Want even more for under $100? Get the latest version, the +100 which includes ATA100 and IDE RAID. You may not need it, but again there have been other improvements on the board. My +IV is so stable, reliable, and overclockable, I'm passing on an upgrade this year. I'll wait till the next generation of P4 (Brookdale) or AMD (whatever runs a Mustang core) chipsets come out late this year before considering replacing this board.
Oopz. I'm a little late. I type like a retard so it takes a while. I think I saw those two boards on eBay, but had no use for one. The +III is still one fine board. Good luck getting one running. But seriously, who cares about the PS/2 port? Just use a serial or USB mouse. As for the the PIII 600 misidentified as a Celeron, some guys are just too lucky!