ok right now i have a single 12" alpine type r, and i was thinking about getting another, or should i get a 18" fi ssd. i have the Kenwood KAC-9104D amplifier. and i was wondering what system would be louder and if that amp would work for either system?
That amp will do 900 rms x1 at 2 ohms. Your sub you have now will need to be a dual 2 ohm sub, get another dual 2 ohm sub then you can run them at the amp at 2 ohms on 900 rms. Theyre rated for 500 each so it'll be close enough. However if you have a dual 4 ohm sub at the moment, this amp will not work for you if you want to get a second sub just like it.
I would go with the 18ssd not just cuz my friend wants your type r, but cuz you'd have a lot of room for the correct size ported box and it would do well with the power you have.