If you're speaking specifically of the base models of the P5K and P5E, and not any of their many variants, the most glaring difference would be the intel chipset each uses which makes a difference on the front side bus speeds and memory speeds supported, among other things. You can see side by side comaparison of the 2 base models
on this link
The differences in each variant board is beyond anything that can be written here. You'd simplify your choices by deciding what your needs are as far as crossfire support, Integrated RAID, CPU choice, etc then see which models support all the thing you're looking for in a board.
I'm not affiliated with Newegg.com, though I spend a lot of money there, but their search options are great for narrowing down the choices that fit you needs.
This link is a great place to start narrowing down your choices. Just check the box/es to select what you require in your board and the search will show you what boards match your needs.
Good luck on making your choice. :thumbsup: