i would think the fosgate may have a lil more sq that the kicker, is there much in the price, i beleive here in the uk the fosgates also dearer! i was gona go for the fosgates but then ordered the 15" l7's.
you could get 2 fosgate t1's for 400 on ebay and a jbl1200.1 to power them perfectly *they are 600 rms subs* for like 200 at cardomain.com. that's 600 for subs and amp. the t1's have 3" xmech, check out the t/s on the website. I heard a set of 15's and they weren't bad for the price I could get them for online, but I'd never pay what they asked at the car audio place.
depends on what you value more, space or SPL. two RF twelves are going to be louder than one kicker twelve, but will take up more space. but sub for sub, the kicker will be louder than a single RF because it has 20% more cone surface.
i don't buy this idea that kickers are not good SQ subs. i use an L7 and it sounds very good. below 60Hz, it's all about moving air, and the kickers do it very well.
as far as build quality, i'm quite happy with my L7. i've owned JL subs before and once i had a spider separation - good sounding subs, but not built the best. i've also owned RF subs - they're okay. my experence with the kicker has been very good so far.