I'd say guards have it harder than centers do. I play mostly at the 3, but I also play a lot of shooting guard, and it's easily the most tiring position to play. When I'm on offense, my coach has me running around the arc, going back and forth, trying to get open. On defense, you have to keep up with the opposing player running around the same way you do. It's very tiring.
I agree, I find that battling in the post, on both ends of the floor, all game long, tires me out much more than simply playing as a swingman, even when I move constantly off the ball like Rip Hamilton. It probably just has to do with trying to muscle guys out of their comfort zone, but I think that would apply to most guys, unless you have some massive weight advantage that makes those post battles easier.
I weigh 150 pounds and find myself battling with guys between 180lb to 350lb. Still find that easier, for me at least.
No offense, but if a guy outweighs you by 200 pounds and can't score on you at will in the post, that guy's not a very good player. Unless you're some freak who's benching 400 pounds, and squatting 650+, he should move you around like you're a child. I've gone against 300+ lb college football players who weren't fat, but they weren't exhausting to guard because their skills weren't great. When I run into a big, strong guy who actually knows how to use his size and strength advantage, trying to stop him from scoring and boarding wears me down like nothing else.
A lot depends on how you play, and how your opponent plays. At it's most demanding, the guard position is tougher. If you play offensively like Rip Hamilton, or have to guard a guy who plays like that, that's about as exhausting as the game gets. However, the guards have a couple of advantages, in addition to the physical stuff that's already been mentioned. First, bigs are the guys who run literally end to end on a possession. They are expected to defend the hoop, and score around the hoop, so they really are going a full 90 feet. Also, the big spots are harder to take possessions off on. Defensively, as a guard, if the ball goes weakside from you, there are several occasion where you will be done, whereas a big is too close to the hoop to disregard a possession like that, and at the very least has to contend for a rebound. Offensively it's similar. It's easier to not crash the boards, or float on the weakside for a possession or two from the guard spot then the big spot.
This is all excluding the Zack Randolf, Eddy Curry, Jerome James group.