Man...I don't know if you can say it "isn't even close", it seems like it's closer than people might think. Although it's tough because the one thing that can make that argument is the fact there's no Edelman without Welker. So it's essentially the whole "chicken or the egg" problem. But in terms of postseason performance and excellence in the clutch...Edelman has to get the edge. Welker was great, but Edelman edged him out in that department. But, again, the thing that gives Welker the win overall is his career totals, along with the fact Edelman's overall success potentially doesn't happen had he not played with Welker to begin with. If he hadn't, I sort of wonder if he ever would have broken through. He gleaned everything and then some, and worked his a** off to do it.
The Hall obviously views overall numbers, so like you said, Welker clearly wins out. But I'm a little tired and I sort of drifted into the tangent of who was better/clutch, etc, which obviously isn't relevant. Hall of Fame-wise, Welker would definitely be the guy.