Upgrading to a vu+ (Ultimo)?

[Rob van der Does]

So how do you compare the 2 ?

Well, frankly: I don't as there's no good way to do so.
You can get some idea of the relative tuner sensitivity if you compare the results on (very) weak transponders, and e.g. the number of services found on weak satellites (provided the satellites.xml is the same, and no networkscan is performed).