Like most things in life, we choose VH's based on how they appeal to us, which is usually based on how they are used - 1 part, 2 part 3 part harmony or more (if the VH is capable) ...
The only person I have heard on this board use multi part harmony to a large degree and with great success is Uncle Dave ... most others use 1 or 2 part harmony with varying degrees of success ...
I do believe that some VH units perform better than others, just as could be the case with any electronic device ...
I agree with Gary in that the proximity of the mic has a lot to do with the quality of the harmony parts - too far away and there could be drop off, too close and there could be distortion ...

On the subject of 'power singers', to ME, Donny comes closest to what I consider to be a 'power singer', based on what I call the timbre, and/or depth of his voice ... Dave and Gary are obviously excellent singers, but their voices - to me - are very different from Donny's ...

IMHO, a 'good' singer should not be singing a song in a key that they could not hit the highest or 'biggest' note of the song without sounding like they are straining, or needing to hold the mic far away, so that no one hears that they can't hit that note ... Some, if not most, of the top performers - the likes of Sinatra, Bennett, Whitney, Buble, etc. all used or use the 'proximity' effect, because they are well versed in vocal and mic use techniques ... and they certainly are hitting those notes ... Of course, THEIR harmony comes from the very talented back-up singers behind them ...

So the 'battle' of VH vs VH will rage on and on, with a lot of rhetoric based on our own bias towards certain brand names, and as with KBs, it doesn't REALLY determine that one is better than another, just how well it works for the individual ...

There's my humble opinion, and it may not even be worth 2 cents ...