I'm going with the sound ordinance. It just arrived today and I test fit it under the seat, it's perfect. If I mount it left/right it won't take up any rear seat foot room, and it i mount it rear/front it only sticks out an inch or so.
Also, the Boss audio unit you linked only accepts line level input, so you will need to split off wires and add a line out converter for another $25 and more complicated wiring.
I read the reviews for all those other cheaper under seat subs from amazon and it seems they have reliability issues. The sound ordinance was only a little bit more and I'd prefer reliability for car audio over cheapness.
It's the best reviewed powered inderseat sub and at $150 it's still pretty reasonable, even comes with the wiring kit from crutchfield so if you don't need to wire in another amp it's pretty much all you need. In addition, it has line level inputs so you just need to splice in the factory head unit speaker outs and you're all set.
Edit: also, nothing boss audio makes inspired any confidence. It's all pretty generic and the general reviews show it. Might as well buy your car audio parts at kraken or O'Reily.