I've only done research, no actual ownership, so I'd take this with a few bags of salt and base more off people with actual ownership.
My understanding:
Zenith is great if you a) want an MP5, or b) want to go hella small.
Scorpion is great if you want amazing performance per dollar, and can't leave anything stock.
Vector is great if you want a range toy with impressive technicalities
B&T is great if you want Swiss machining, without quite as large a price tag as it normally runs.
MPX is great if the Vector wasn't heavy enough for you.
I've handled the Scorpion, Vector, and MPX carbine/MCX pistol. Out of those three, the scorpion is far and away my favorite, but I'll give a little bit of a breakdown on each one. Below here is only dealing with these three, and no regard to the Zenith or B&T.
While the MPX did feel extremely well made, it also felt nearly as heavy as my AR15. If I'm getting something that size and weight, I'll get an AR pistol in 5.56 or 300blk and have massively superior ballistics. Or I'll just build an AR9, come in cheaper, lighter, and more personalized.
The Vector was nice, but between it's weight, balance, ergos, and online reports of questionable reliability, I wouldn't pick it for serious use. However, the Vector is the only one I've fired, and there's effectively 0 recoil. If I wanted to run a PCC competition, this is honestly what I'd probably pick. You do have to deal with an insane height over bore for your sights, but between taking glock mags and staying on target like a laser beam, this one really is a gloriously entertaining gun to shoot at the range.
Overall though, I'd go with the Scorpion. For three reasons:
1) Price for performance and fun, the Scorpion seems to be out front by a good margin.
2) Ergonomically, the MPX is better, but if you replace the safety on the Scorpion then it's neck and neck, if not ahead.
Which leads into:
3) The number of modifications available for this thing is (correct me if I'm wrong) one of if not the best on the market for PCC's.
As someone who can't leave a single thing I own in stock condition, this last point is an amazingly important thing. As someone who's college-level broke, the price point of the Scorpion and it's available parts is very attractive. I haven't heard any reliability issues like I have with the Vector, and it doesn't feel like a boat anchor as the Sig does.
Here's a quick and interesting thing on the weights, and why I can't stand the Sig:
Sig MCX pistol: 7.4lbs w/brace
Sig MPX Pistol: 6.2lbs w/brace
Sig MPX Carbine: 7.6lbs
CZ Scorpion Carbine: 6.4lbs w/muzzle brake
CZ Scorpion Pistol: 5.0lbs w/o brace
CZ Scorpion Pistol: 5.8lbs w/brace (added on afterwards) (sb tactical brace is 12.4oz, which still keeps it sub 6lbs)
The Sig MPX pistol weights near as much as the Scorpion carbine, whereas the Scorpion pistol is nearly a half pound lighter than the lightest MPX with both braced. I'm not finding actual numbers right now, but I'm assuming you could cut that weight down even further with a Tailhook. If I'm getting a PCC, it's because I want something lighter and more compact than my AR. When I can build or buy an AR that's as light or lighter than the Sig PCC's, I fail to see their purpose. There's simply no need or reason for them to be as heavy as they are. Especially at their high price point.
TL;DR - the scoprion is nice and light with a really good price point and a wonderful aftermarket. The MPX is a boat anchor, the Vector has a few issues to keep it from serious usage, and I don't have direct experience with the Zenith or B&T.
Currently, if I wanted to just go get something right now for most any use, I'm going with the Scorpion. If I wanted to step up in price slightly to get a go-to-war PCC, and don't mind waiting to source one, I'm probably going to get the B&T.
edit: again, I have only fired 2 mags through the vector, and just handled the mpx/evo. If I'm wrong on any of my above statements, please correct me.