well they are the same just have to wire them differently. If your getting 2 id get D4 it allows you to wire them to a 1 ohm load thus saving you money on an amp.
Personally i would look into buying a new amp, if your on a budget look into Hifonics and get a 1210d or an older model like the 1206d. Get a nice 1 ohm stable amp that can push 1000 watts at 1 ohm and you can get those things moving.