I use both. What's the difference?
They are made by different developers and will vary in terms of features and underlying blocking mechanism.
I am not very familiar with AdBlock Plus, as I’ve always used AdBlock since it was available. Some people will undoubtedly claim that one is faster or less resource-intensive than the other, but that is something you have to ascertain for yourself. That being said, I think that AdBlock is a little easier to configure.
Using both would be waste of resources however, as they will probably use the same block lists. Just pick one and discard the other, they will both probably achieve similar blocking results.