Like I said, SC2 could have more custom models then war3, but like Rui mentioned, war3 will probably have more diversity. War3 has creeps, which allows a lot of different races of creeps (trolls, gnolls, dragons, revenants...). SC2 Doesn't have creeps thus, most units will be Zerg, Terran or Protos oriented, except for critters.
And about 2 expansions, as I understood, they are mostly there to split the campaign. I don't really think they will have to big of an impact to map making, unlike TFT.
Again, I hope I am very, very wrong, but logic points to that direction...