You know what, I'm studying this a little closer. I know Glavine pitched into his 40s to get to 300 wins and is a "compiler" in that way. One could argue his rate stats, compared to Mussina, were hurt by pitching for so long.
So if you stop Glavine's career at age 38, therefore, pitching the same amount of years as Mussina, you find:
Glavine: 262-171, 3.44 ERA (121 ERA+), 1.29 WHIP, 65.3 WAR
Mussina: 270-153, 3.68 ERA (123 ERA+), 1.19 WHIP, 82.7 WAR
They look like very similar pitchers during this time, aside from the WHIP and WAR. I imagine that those 2 stats are correlated in some way. Glavine's FIP and xFIP were significantly higher than his ERA, because of the way he pitched. He lived just off the plate, with the idea that any contact that is made isn't hit hard. In only 7 of the 18 years Glavine pitched did he allow 20 home runs or more, which is fantastic. He walked a batter more per 9 innings than Mussina, and because he wasn't a K pitcher, he allowed more contact, and thus, he allowed more baserunners than Moose. But again, his goal was to put the ball in spots where it was hard to hit the ball square, and that's why his ERA didn't suffer as much as you'd think it would considering his WHIP.
So while I'd argue by looking at the above that Mussina was a better pitcher during that time, I understand the argument that Glavine was more often considered the best pitcher in his own league year-to-year, and because he pitched longer, he showed more durability and accumulated more wins and innings. The more a great pitcher pitches, the better his career is. They are neck-in-neck in their careers, IMO.