two 12" type R would be much better than one 15" type R.
if you want to wake up your neighbour's neighbour (:P), than i guess you will need a bigger box that is well designed for two subs. i remember seeing kevin's box for his two 12" type Rs and that was amazing and pretty loud.
if space is not a concern, you cud get two 15" type Rs ...but i dont know which car you have, because two 15s wud need a very big box ...plus very expensive. two 15" will cost u about 30k or so...two 12" wud cost you around 18k i guess.
but kicker wud be louder than anything...if ur ultimate concern is loudness. get kevin or maggi to design you a box with super loud tuning
my emphasis on box is because it can make or break a system.