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!