Just pick one and be happy. The Mach 5 12 is cheaper, smaller, and can handle about the same amount of power and probably has better SQ. The BTL 15 is bigger, more expensive, can handle maybe a little more power, and is more customizable.
Ultimately, since you have $250 shipped, you can't get either of those anyway, so useless thread.
If you want a loud sub for $250 shipped, you can get a RE SX 15, Fi BL 15, couple of Fi SSD 10s, etc, etc, etc. There are tons of options. What it comes down to is how much money, space, and power you have. Along with endless other factors.