Much of the discussion since yesterday's announcement that Carlos Condit would be replacing Nick Diaz against GSP in the main event of UFC 137 centers on the premise that what's lost promotionally by the switch is made up for Condit being a tougher challenge particularly for the champ.
Is that really true? Under some key considerations, yes, although the truth is the threats are roughly equivalent over the course of a five-round fight.
Standing, Condit does pose a more significant test than Diaz. He possesses excellent power, pocket fearlessness, a more complete kickboxing game, a longer reach, controlling clinch and most importantly, is trying to score or damage opposition continuously from bell to bell or in every inch of space. Diaz's accumulative style of damage and sole reliance on boxing isn't nearly the threat on the feet as Condit's aggressive, even borderline reckless attack.
There's an open question, though, about the amount of time the fight will be spent in that portion of the game. It is true Condit and not Diaz would be willing to drive a knee into the body or chin of a level-changing GSP hunting the takedown, but it's also fair to argue that's a fairly narrow window of opportunity. Condit's an excellent fighter and a good athlete, but has demonstrated some measure of difficulty dealing with the speed and strength of superior athletes.
Either Condit or Diaz would be expected to spend significant portions of the fight off of their backs. Evaluating them there is a touch difficult. In pure jiu-jitsu, Diaz is the far more accomplished and complete talent. In terms of his ability to use that jiu-jitsu in MMA contexts, we haven't seen much recently at least not against top-level opposition. Over the course of his career it's true Diaz has demonstrated excellent and aggressive guard play. However, since taking his talents to EliteXC and then Strikeforce, he hasn't been forced to use them against anyone reasonably considered high level.
Some will point to Condit's improved ability to sweep and attack from the bottom. That's true, in part. Against Kampmann and Kim he showed excellent upa or hip bump sweeps. He took or and often attempted the taking of the back against Jake Ellenberger, especially late in that fight. But in virtually all of the cases save Kim, whatever gains he produced from achieving dominant position were somewhat offset being unable to control or maintain often due to overly aggressive attempts to finish. This was particularly true in the Rory MacDonald fight where the Canadian was able to both achieve dominant positions and reverse out of bad ones because of Condit's loss of control through aggression (up until the third round, anyway). Moreover, if Condit will need sweeps to accrue enough points to win or enough opportunity to do damage, he's going to find St. Pierre's base on top is an entirely different challenge than that of Kampmann or Kim.
If we are discussing who possesses the guard that's more difficult to pass, it's virtually impossible to argue Condit's is superior to Diaz's. Diaz would likely spend much more time forcing St. Pierre to work from guard. Condit's guard was passed multiple times by Rory MacDonald. One can only assume GSP would be equally able to achieve dominant position past the legs and hips of Condit.
In terms of durability over the long haul (and GSP fights tend more than not to be long hauls), Condit and Diaz are both difficult to finish. Diaz and Condit both absorb more damage than advised standing, although Condit has been placed into more precarious situations than Diaz as a result. You'll recall some scored the first round of his fight against Ellenberger a 10-8 after Condit was reeling from hurt by Ellenberger's punches. Diaz has been hurt as well, but typically recovers more quickly. The issue, though, is who is more durable over the course of five rounds. Condit is put in bad spots and often early in fights, but demonstrates gameness late. His style of fighting is very back and forth as well as taxing, but I'd offer he prefers it that way. Diaz, by contrast, possesses a strong chin, no longer cuts as easily and can absorb tremendous body punishment for long periods of time. Both have excellent gas tanks. It is arguable the difference between the two, then, is Diaz's generally less taxing style and presumed decent control off of his back would make him a fresher fighter late.
Then again, if he's more content to wait for opportunities to come to him even in championship rounds, maybe Condit's aggressive style - for all its drawbacks - is slightly more challenging for the welterweight champion.