PPPoA is the setting to use in the UK.
PPPoE is for use in other countries where there is no ATM section of the network (which is what the A in PPPoA stands for).
In the UK, BT have installed PPPoE emulators so you can log on using PPPoE if your router only supports that, but you will see a very slight performance decrease from PPPoA.
Basically, if PPPoA works, that's the preferred connection type.