I have t-sliders on my van (2 on driver's side, 1 on passenger's side) from CR Laurence. (There are a few photos in my gallery.) I think it's the only regret I have about my design, because of the rain issue. If I could go back and do it again, I would definitely go for the awning-style windows. In retrospect, I didn't give this enough thought during my design process - I went for sliders under the impression that they would have a larger opening than awnings, but seeing Greggde's photos, I think that is misguided. One positive aspect of the sliders is they are (probably) quieter if open while driving.
I am going to look at switching out the two rear windows to awnings, but don't know if there is a size available that would match (or slightly enlarge) my current openings without affecting the inside layout. BTW, I was very conscious about spec'ing the rear windows to be symmetrical in size and placement, and designed the interior based on that.
Cheers from Vancouver, where it is currently raining!
Brent