It is less that I think Talib is overrated and more that I think you are underrating Rodgers-Cromartie. He is a good CB, and had a damn fine season in 2013.
Over the course of their careers DRC has been a better CB than Talib and that is not even debatable in my opinion. His first 3 seasons in the NFL he had 61 pass defends, 13 interceptions,and he returned 4 interceptions for a touchdown To put that in perspective Talib has 70 pass defends in his 7 year career.
As far as Talib goes I am not arguing that he played very well in 2013, for the first 5 ½ weeks of the season he played like a top 3 CB. That said 5 ½ games does not make you a top 10 CB and if you look back prior to the season the general concensious was that Talib was not even a top 15 CB among many fans and posters in here.
I would be curious why you feel Talib is better than DRC. The numbers do not reflect it, and if you watch their tape I do not see it. It cannot be talent because DRC is arguably the most physically gifted CB in the NFL. The guy is an absolute freak in terms of athletic ability, he is 6’2” 190lbs. runs a 4.29 forty and can jump out of the stadium.
What numbers don't reflect that? Cornerbacks are notoriously the hardest position in the NFL to evaluate based on stats. Part of getting a pass defensed is that the opposing QB must have thrown at you. If you've really locked down the receiver, typically the QB won't even throw to him, and thus the statistics will show that you did exactly as much as someone sitting on the bench did. In 2010, Darrelle Revis had passes defensed and 0 INTs. Did that make him a bad CB? Of course not, he was the consensus best CB in the NFL at the time, even coming out of that season, and nobody thought even one iota less of him because he didn't defend enough passes.
Talib has played in 77 games, in which he's defended 70 passes (0.91 per game) and intercepted 23 balls (.30 per game). DRC has played in 92 games, in which he's defended 98 passes (1.07 per game) and intercepted 19 balls (0.21 per game). Their stat lines are very close, if you choose to evaluate them, with DRC's edge in PDs countered by Talib's significantly greater edge in forcing turnovers.
This is before accounting for several factors, starting with the fact that injuries--and the fact that Talib has played a huge part of his career with them--make this comparison closer than it otherwise would be. My initial point was and always has been that the injuries close the gap quite a bit, and your counter was that even when healthy, DRC is better than Talib. Yet if you choose to evaluate them by passes defensed and interceptions, then it's at best a wash for DRC even before you account for Talib's injuries.
I still haven't addressed your question, though, which is how do I evaluate them. The answer is basically the eye test. I watch them play, since, again, CB is a position where stats aren't of much use. I couldn't care less how big and fast DRC is, because watching him play makes it clear that he's a soft player. He's talented and athletic, and he leverages the gifts that he has to be a good, above average starting NFL CB. But he's not going to disrupt anyone's gameplan week in and week out. If he's on your #1 receiver, then he's not going to force you to abandon your gameplan- your top receiver is going to get his stats. Just look at last year:
Week 1, Torrey Smith got 92 yards. Week 2, Victor Cruz had 118 yards and Nicks had 83. Week 3, Denarius Moore had 124 yards and a TD. Week 5, Dez Bryant had 141 yards and 2 TDs (and Terrance Williams had 151 yards and a TD). Week 6, Blackmon had 190 yards. Week 12, Edelman had 112 yards and 2 TDs. The effect became less pronounced as the season went on and the Broncos played a bunch of teams that couldn't pass, but more often than not, the top receiver got his stats on the opposing offense. That falls in large part on the CB1's shoulders.
So yeah, DRC is good. When Talib left, I advocated for signing him, because even though he's soft, doesn't tackle, and can't play press coverage to save his life, he's still a very talented, extremely athletic football player. And when you account for Talib's injury risks, which discount his value by a large amount, you're left with a similar tier of corner. Talib is much better when healthy, but he's not healthy all that often. When he is healthy, though, he's better for a few reasons, starting with the fact that he's physical, he tackles, and he can jam the WR1 at the LOS and disrupt the timing of the offense. He also plays the ball very well, and makes sure that when QBs do throw to him, they're punished for doing so.
And again, I'm pretty much positive that there isn't a single poster on this forum, yourself included, who would have claimed two months ago that DRC is as good as a healthy Talib. Most would argue that it wasn't even all that close.