A commonly asked question.
5150 II/6505+ (same thing, different name) are slightly less balsy, more upper-mid focused beasts with separate channel EQs, an improved clean channel, foot switchable crunch (pointless), bias points (pointless)
The 6534+ I've never played but it is the same feature-wise on the outside as the +/II, it runs a quad of EL34s and therefore has a more British flavour, i believe it has a better clean channel and a noise reduction circuit in the gain channel.