I'll say on Suh.
From 2009 to 2010, the Lions...
-Won 4 more games
-Allowed a full touchdown less a game
-Sacks went from 26 to 44
-Yards allowed per game went from 392 to 343
-Defensive Qb rating went from 107 to 89.1
-INTs went from 9 to 14
And I'll reply with
From 2009 to 2010, the Patriots...
-Won 4 more games
-INTs went from 18 to 25, 1st in the NFL
-QB rating allowed dropped from 81.7 to 81.2. I know it's not a big difference but this defense and secondary is very young and had growing pains.
I also think it's a bit illusive to credit Suh with the passing stats and not look at the rushing stats.
2009 Lions 126.6 ypg 4.4 ypa
2010 Lions 124.9 ypg 4.5 ypa
So although the 2009 Lions gave up a couple more yards per game on the ground, they allowed less yardage per attempt than the Lions with Suh. It's pretty safe to say that the ground defense has hardly been impacted by Suh's presence.
To sum up:
Suh's present resulted in more sacks on QBs.
He didn't affect the run game much if at all. That aspect of his game is pretty disappointing for a DL. I'm sure there were other parts of the Lions that were bad too so the blame isn't entirely on him. But neither did he 'raise' the level of play of the Lion's overall run defense.
McCourty's presence vaulted the Pats to #1 in the NFL in INTs and 1st in turnover ratio. He only helped lower the overall opponent passer rating a smidgen.
So now you have the quandry of which came first - the dominant passrusher or the dominant DB. Each affects the game in different ways. But consider that 7 INTs are definitely 7 turnovers. 10 sacks are not equal to 10 turnovers. Both are very valuable defensive players and you can make a compelling argument for either one of them.