There's a lot more space behind the seat really if you build a long box. In short under seat boxes aren't great unless custom anyway, and a custom seat behind the seats will be easier to build than the under the seat. Porting under seat boxes get's difficult as well due to the short dimensions. I guess same size box/sub/amp, won't be a huge difference, but as I mentioned, behind the seat will give you more options such as porting a box and such which can yield a louder setup. Under seat boxes are suitable when your expectations aren't very high.