In general, when you have more than 2 sticks of ram, you also have to double the command rate from 1T to 2T in order for it to work. I would say that the 2 sticks would be faster, as well as giving you upgrade possibilities for the future.
On some motherboards (I think it was 1366 motherboards) there was a note that using a single bank of RAM was somewhat faster than using two. That would suggest that 2 x 4GB would be faster than 4 x 2GB. From what I have seen, 2 x 4GB is often slightly cheaper, too. And as clarkjd points out, it leaves room for later expansion. Sounds like 2 x 4GB wins each round
Yeah I too original thought filling all the slots would increase speed but I read that it's better to have just two sticks of RAM for the following reasons:
less strain on memory controller (faster)
less chance of memory module failure (twice the # of modules, twice the chance of failure)
Leaves options for any needed memory upgrade in the future