I have and play both ....
Starcraft II is a good example of a very polished game. The campaign is well done and everything works as you expect. That said, it is not much different from the original. At the end of the day, despite probably one of the strongest community and MP interfaces out there, the game itself is a typical 4x RTS .... and I do like it.
Elemental has some potential but is currently an unpolished gem. Version 1.06 is still quite buggy and the AI just plain sucks... Despite its problems, I am enjoying playing it, but my jury is still out regarding whether it will have permenancy here on my hard drive.
I actually stumbled on the Dominion 3 as a result of a post in the Elemtental forums...favorite Fantasy Strategy games to play while Elemental is being fixed..... D3 got a lot of play in that thread and I ended up buying it. I am just scratching the surface, but I really like what I am seeing here despite its dated graphics. It is definetly much deeper than Elementals War of Magic