3rd alternative to this all, which i happen to like:
thoroughly educate yourself in the matter at hand, car audio in this case. buy gently used equipment from a respected/reputable enthusiast through a forum or ebay at a significant discount. take the time to do a proper install (design your sub-box with a CAD program, build MDF speaker spacers or pods, sand away paint and ground your amp to your car's chassis, etc). you'll end up with some useful knowledge, a feeling of satisfaction, extra money in your pocket, and a better system for the money than your could get elsewhere.
not trying to stiff knowledgable/helpful retailers, but i trust myself more than you!
