The latter is way, way, waaaaaay more popular than Mark, like you it or not.
I'm not an MK apologist but I'm not sure if DG is that much more popular than MK.
A crude measure - Spotify monthly listeners:
Pink Floyd: 18.7m
David Gilmour: 628k
Dire Straits: 17.7m
Mark Knopfler: 1.9m
Good point, which of course, tells you about the popularity of the music, rather than the popularity of a personality. Mark is a more consistent and more prolific songwriter than David, let's face it, but David simply has to be more popular than Mark, no way it's not the case. Just walk outside and ask a random spectator if they know who David Gilmour is or who played guitar and sang in Pink Floyd, and what about Mark Knopfler? It's even difficult to pronounce the guy's name, let alone remember it!
I don't know about other countries but here in the UK I genuinely think it would be about even.
MK had "guitar god" status and DG, while highly regarded for his tones and originality, never quite got there.
All thanks to "Sultans Of Swing". Mark owes his whole life and career to that one song, without it he would be a niche musician and not a "guitar god" by any means. If we talk about Sultans, then yes, I'd say they are equal, but everybody seems to be obsessed with David's solos, you see kids everywhere playing "Time", "Comfortably Numb", "Shine On", "Marooned, and "Money" solos, Mark has nothing at all like this, only a bunch of people playing one song over and over again, and completely avoiding his entire discography.
Still, no way on this planet Mark's guitars are gonna raise 21 mil like David's, I think anybody can agree at least on that. And you likely won't have Jim Irsay buying a single lot for 4 million either. I'm curious to see the results and want to be wrong, but unfortunately, experience tells me it will not be spectacular.