I would say blitz, they can both think and move faster. The longer the game goes on, the more it levels the playing field. I don't think it ever gets level. The reason is, even if you gave a GM weeks to try to calculate everything,  I believe the human will err before the machine does. Others here see it differently, though there aren't many records to back this up. I will just tell you that when I play certain GM personalities on Chessmaster, I can give em a game for a while. When I try to play the brains behind the engine. It cuts me to pieces fast. Chessmaster isn't as good as Rybka or Houdini. Chessmaster's ELO is about 2850, Houdini 2.0 is like 3300. I just don't see a GM rated 2900 beating Houdini in the standard length of game that the world championship is played at. Some people don't give the computer program enough credit because it can possibly be beaten. If you took the winning percentage of Houdini 2.0 at any time mode against a GM, I bet it is comparable to a super GM or better. We would have our hats off to him, but not the ability of the computer.

I think that though he could calculate everything pretty well so many moves out for a good deal of time, I believe Houdini 2.0 would be able to calculate more moves out and for longer without flaw. This is why I think Kramnik fails eventually with that time mode.

Chess is about more than just calculation though. The longer the time controls, the less important tactics and calculation are (a computer's strength), and the more important strategy is (a human's strength).

I disagree on the basis that the more simplified the game gets, the more it is about shear calculating ability.Computers are programmed to take advantage of end game tables.

Strategy, especially against a computer is more important early on by doing things like, purposely playing a closed game and making sure to try to trade one of your pieces to leave the engines only bishop to be the opposite color of what your pawn chain and king and stationed on. In doing so, if you manage to atleast keep even material, you reduce his bishop to about the value of a pawn.

Also I have been told that computers aren't sometimes programmed to deal well with pawn storms that attack a castled king on his own side. I have personally tested both of these theories against my Chessmaster. I did pretty well against GM Bogoljobow's personality, using the first of the two strategies I mentioned, but I still get cut to pieces by the engine's brains itself, no matter what I try. Rybka and Houdini are superior to Chessmaster's engine.

I don't think I said anything about endgames, but engines are supposed to play them weakly since they are very strategic. This doesn't apply of course to endgames with few pieces where an engine can play perfectly.

computers play endgames weakly because pure calculation fails in many endgame positions. this is why databases are used. computers have a major problem with fortresses. they have a horizon effect since many games it takes 30+ moves to convert a win. remember computers calculate EVERY move. Ra1 ra2 ra3 ra4 etc.... which in endgames the positions have too many useless options

This simply isn't true. Computers couldn't calculate nearly as deeply as they do if they looked at every line, unless that wasn't what you meant and I misunderstand.

hence why endgames suck for computers. they try to prune them but endgames are hard to calculate because of certain maneuvering Ra2 might be the best move over Ra1 in a given position but there is no concrete reason why until move 20. In middlegames there are some tricks they use based on looking at forcing moves first etc 

Engines don't always find the best move. I had an engine tell me once when I was sacking an outside passed pawn to divert the enemy king that I should have promoted to a bishop instead of a queen, when the position 1/2 move later was the same either way.

And then there's games like this from older engines (from NM aww_rats):

I'll agree that newer engines are much better and quite possibly unbeatable by humans, but they still don't understand openings or endings, and shouldn't be used to measure which ones are best. In the KID (and benoni as said before) the engine algorithms say that white is winning because the algorithm likes the extra space. Then, when black kingside pawn storms (also mentioned above), it suddenly changes its mind and realizes that black has good chances too. The algorithms also like gaining tempi more then they should, they will undevelop pieces to gain a few moves. This, sometimes they completely misevaluate positions.