Both from ikesound would be about the same price buttttt with one W3004SPL i'd have to spend more on a box because i want it to be ported and what not.
the 3004spl OWNS...they're both well built subs, but the 3004spl with be considerably louder in a ported enclosure...i have the 10inch version of the 3004 and its loud free air, havent gotten a chance to build a box yet
I tried both setups off my JBL Gts600 both tuned to 33hz... the 3004spl in a 2.5cft box and the 306dvc's(same 126dvc) in a 3cft box and both were great but the 2 306dvc's got a little louder... but if you want a single sub setup the 3004spl would be a great choice...