Fundamentally the same board as far as compatibility is concerned. I rather like the SiS P4 chipsets as compared to the early, brain-dead Intel ones with bad memory (i845) and PCI (both i845 and i850) performance, but the 865P(E) is pretty much the ultimate P4 AGP chipset, both in terms of performance and compatibility. But that means it'll run Win98 without issues. DOS wil run fine on the board itself, but drivers for cards are the issue, as well as raw speed (far, far too much of that).
Big "but" though on the Epox: Epox boards are notorious (even by the standards of the time) for awful capacitor quality. Be prepared to replace pretty much every cap on the board if that hasn't already been done. Fortunately they were also pretty much the best enthusiast boards at the time, so it's worth it - although Gigabyte boards are generally also well supported, fully featured and stable, so If the Epox is in the state I'd expect it to be in and you can't (be bothered to) replace the caps, it's a perfectly good second choice. Check its caps too, Gigabyte tended to use better ones, but after 15 year's that's no guarantee.
If you have a P4 system and a P166MMX and want to use one for pure Win9x and the other for Win9x and DOS it's a no-brainer: the P4 865PE system is the ultimate Win98SE system (er, barring some AMD stuff 😉 ), the P166MMX is far better suited to running DOS stuff, both in terms of speed and availability of ISA slots. It can also run Win98SE if needed, but will give you more of a "period" experience there 😜
Note: in both cases, take it easy on the RAM. The P4 (or rather: the 865PE) can handle 4GB, but Win98SE doesn't like more than 512MB (yes, there are patches, but unless you're doing stuff you could better do on a modern build, there's no reason for anywhere near so much). In the case of the P166MMX, the chipset and motherboard cache config determine the optimal amount of RAM. Most CPUs like that will be paired with a board that can only cache 64MB. Anything over that will incur you a big performance hit (unless you are actively using more memory, in which case it's always faster than thrashing). Check which motherboard that CPU comes with to be sure, as there are exceptions. If you're lucky it could be something like an Asus P55T2P4 rev 3.1 with second tag RAM installed. In that case it can cache 512MB - if you can find the 128MB 72p SIMMs you need to get there 😉