I also tried the SL1, and found the same problems with it you did. I also found one more: the door angles back, and when the vestibule is open, it doesn't overhang the entire door. As a result, you do get water inside the tent when it rains - not much, but enough to be annoying.

The Hubba vestibule does overhang, so the tent interior stays dry. (This is the wider part of the top Jason talks about below.) The overhang doesn't really create any extra width inside the tent, but it does a good job of keeping the walls near vertical and keeping the water out.