Well, Snap On, as I understand the situation, bought out the old Vacuum Grip company many decades ago, which is how their plier line came about! Those were top grade pliers, and no less so for Snap On's takeover! Indeed, some of the original 'Vacuum Grip' models were still offered up till the late 80's - I bought some!
Back then, those all steel Vacuum Grip models were really the top end of their line, but their regular models with the dip coated handles were pretty darn good too. I bought some of them as well, and am still using them! If there was a downside it was that Snap On seemed wedded to producing 'traditional' patterns and never offerred a European style combination plier, despite it being my most used pattern!
Sadly, over the years, it seems to me that the quality has declined a little. The new 'Talon Grip' were launched with much fanfare, and I'll admit they are decent tools, but the more complex joints of the older models have given way to an (easier to make) lap joint, there are many more nicks and dinks in the forgings than on the older models, and the finish is poorer.
Having said all that, they are still very good tools. I bought a set 'on special' for my road box and they are lovely to use! The long nose pliers have taken abuse without issues, and I reckon the cutters are about the best out there at the moment!
Knipex are an average to good European manufacturer. 30 years ago, I'd have said that they didn't have the fit/finish of the Snap On, or the strength. Of course, they didn't have the price tag either, being half the price of the Snap On's, or less! They didn't sell big numbers in the U.K. owing to the proliferation (and quality) of our domestic brands!
Things change though, and while incompetent British managers fell over themselves in the race to bottom, the Germans kept the quality high! I would say that Knipex are probably now the predominant plier manufacturer in the universe, and certainly in the U.K.
Their quality is very good. They have probably overtaken Snap On now in terms of quality of fit, and quality of finish, but I think that Snap On still have the edge in terms of absolute strength!
Ultimately though, it all comes down to personal preference, and there can be no substitute for trying a pair in your hand! Despite the Knipex being superb value in the U.K. I still reach for my Snap On's first!