That my friend is a tough question. The show I just saw saturday featured both subs in multiple cars and they metered almost the exact same (1-2 db difference). I owned an Fi SSD fully loaded and loved it. The build quality was second to none. I currently own a SoundQubed HDC3 18, but haven't installed it yet. I can say without a doubt that my new SQ sub is the stiffest sub I've ever purchased. The quality looks amazing and the guys I talked to on saturday just raved about their SQ subs. I don't think you can go wrong with either to be completely honest. If it were me, I'd go SoundQubed. They are cheaper and shipping is free. Plus, your sub will arrive much quicker.
just noticed you own an HDC3 hahahaha so basically ignore everything SQ related above haha