Its a matter of physics, not opinion. In practice though it might not make a single bit of difference, especially when you count in all the little holes through which air can enter or exhaust, depending on set up, if you know what I mean.
I plan on getting the H100i as soon as funds allow after Christmas. Then I plan on doing an extensive test to prove my theory right or wrong. Right now I'm betting my negative pressure will prove a couple of degrees cooler than positive pressure, as the air heated by the motherboard wont be passing through the rad on the H100i
On a similar note, I saw a very intersting youtube vid regarding the MSI Z77A MPower, and why MSI makes it the way they do.
http://www.youtube.com/watch?v=kBGg_-1jTJg
The whole video is pretty interesting but the specific part I'm refering to starts about 16 minutes in, so skip forward to the part where they talk about the heatsinks on the board and water cooling
