The p3v4x also has the advantage of 4 dimm slots. You can use all 4 for up to 2 GB (in theory) total RAM, all banks running at PC-133. The i815 has only 3 dimm slots and a maximum RAM limit of 512 MB. What's more if you use all 3 dimm slots the board reverts to PC-100, so you really only have 2 if you want PC-133.
Someone already pointed out the cost issue: via 133a is less than i815.
Only other difference is that the p3v4x can run your memory bus at 133 even when your fsb is at 100. The i815, while having other asychronous memory options, can't do this particular setting for some reason. It's no biggie as few will ever need this setup.
So I'd recommend either board. It may turn out the cusl2 is more stable and has fewer quirks than the p3v4x but only time will tell.