Based on the fact if you go to the Asus UK site the Pro isn't listed, I'm guessing that they are essentially slightly different versions of the same motherboard intended for different markets.
The Pro has some slight improvements in a few areas, namely extra power phases, some extra USB3.0 ports etc, but there really isn't much between them.
The Plus is fairly reasonably priced if you want H87, ATX and an Asus board, but the Pro is definitely too expensive. If you are happy to compromise on any of those three qualities, you can save money.