Which team has the better offense?
I think that Dallas does. Nash offenses tend to be very efficient, and outside of Josh Smith, Dallas has a lot of efficient players. Nash, Gordon, and Deng are all plus-shooters, and Big Al is a huge threat in the post. Nash, Gordon, Deng, and Smith are all excellent passers. If Nash can keep Smith in check -- and I think that he can -- this will be a beautiful offense to watch.
OKC, meanwhile, has several below-average offensive players in Gibson, Budinger, and Shumpert.
Which team has the better defense?
For this particular series, I think that Dallas does. Smith and Deng are elite defenders; Gordon is a very good one. Big Al under normal circumstances would be considered below average, but in terms of his matchup with D12, he plays him evenly. Nash of course is a below-average defender, but probably no worse than Budinger.
Which team is better at rebounding?
Pretty clearly, this is Dallas, right? Big Al and Howard are generally even, but Smith + Deng is going to add more rebounding than Gibson + Budinger.
Dallas has a very good offense, a very good defense, and very good rebounding.
OKC has a limited offense, a good defense, and good rebounding.
Advantage, Dallas.