Differences between various Quantum devices

I realise there has been some discussion of Quantum QX4 and QX2 devices. I have seen them at several shows and am sold, personally, on the gains envolved. They are not cheap, but I am all ready to pay for a QX4 and I find new alternatives. Firstly QRT themselves are producing a QV2, a small plug in device for sockets and conditions. This evidently is not the same as QX4 and 2, it uses different technology. Kemp as well have the much cheaper plug in QA plug. So what do I do know?

A big ask I am sure, but has anyone compared the differences and merits of QX4 or 2, with the QV2 and Kemp plug. I would prefer the latter if it works as well, partly for the cost, but also it means I don't have to find room for another shelf in my rack. Thanks David