the olympus is a pretty solid woofer as well.
although i don't think we can tell you the answer, since there hasn't been a direct comparison.. I'm using a pair of 1615's right now and i was surprised by them.
i can tell you the olympus will naturally sound better. I've heard both hdc3 15's and owned 12's and wasn't impressed by either, as far as how they sounded. although they sure did get loud
i think i am gonna go with the hifonics they look pretty well built and i can get them for a good price check them out
http://www.hifonicsusa.com/Hifonics_Olympus_Subwoofers.html