One of my gripes with the Slimline (I don't own it, but am seriously considering it) is that it seems to sit sooo high, partially due to leaving room for one of their fancy table slides between rack and roof.
BUT... I talked to the company directly, and it turns out you CAN buy an alternative means of mounting that skips the single-piece-per-side rail, instead using more traditional mounts that CAN sit lower. If I go with the Slimline II, I'll opt for that route to get a lower profile.
Funny, the solid piece mounts are one of the main reasons I'm leaning toward the Front Runner.