Not being a bass expert here is what I think.
Firing into the cab will be louder but yeah it does have a tendency for cancellation.. It has more to do with front speaker placement and the specific vehicle though. Firing to the rear isn't quite as loud, BUT it is what people call corner loading and extends the low end. Corner loading usually does make anything loose rattle or buzz. If you take the time and leave yourself enough room you can use damping material to eliminate most of it. Without modification though it is nearly impossible to get rid of all of the rattle. In places where mat won't qite go try spray on damping material or if its a space that needs a lot try something like spray expanding foam to fill the hole, being extremely carefull not to over do it because it can and in some cases has bulged panels inside and out.
I was thinking about your box design. I don't know if its a symetry thing I have going on or what but I would angle the sub to the same angle as the panel it is going to face. angled up for the back seat, or verticle for the rear of the vehicle. You could also downfire it but you loose a bit more volume. It does however make the bass much harder to localize, and you would have less trouble with rattling panels in the hatch area..
Your friendly neighborhood subaru freak!