Obviously a plate amp makes it a stand-alone unit. But the enclosure will be easier to build w/o having to mount up the plate amp. Also, what's a good regular sub amp
Plate Amps are pretty cheap. Plus to make a hole in the enclosure you just use a Jigsaw. Even a $20 black & decker model can make a square cut if you just predrill a hole.
I like working with plate amps because you don't need a speaker terminal or speaker wire, just connect plate amp wire to driver. heh
A standalone amp would probably be better though... Box is stronger without amp mounted in it. More upgrade friendly... just more costly.