It depends. If money is a factor and you want a high performance sub I say DIY and passive.
I have three passive subs and a stand alone amp for those cost less than a decent active sub, and the performance is waaaaay better.
Big Bertha is a ported Dayton Ultimax 15", around 190 litre.
Small one is a closed 115 litre Dayton Ultimax 18"
Closed eats up a lot of watts, but a Behringer EP2500 works for two of them and is cheap