so i have a single kx1200.1
300rms @ 4 ohm
600rms @ 2 ohm
1200rms @ 1 ohm

i have a single L7 15" that is DVC 2 ohm
i can get a nother L7 15 that is DVC 2 ohm

with one sub i will be at 1 ohm and 1200rms
with two subs i will be at 2 ohm 600 rms

which will be louder?

 

the kx1200.1 is 1200Wrms@ 2ohm and will produce 1200@1ohm.but its touchy
at 1ohm.
iv had a few of the KX1200's and the KX2500's
they ran very well in 2ohm.but iv blown a few running in 1ohm.

now i have the ZX2500.1 amps

 

Personally, I'd say more cone.

 

that 1200.1 is actually rated @ 1440 RMS and it is 1 Ohm stable. (According to Kicker)
If i'm not wrong the 15's are 1000 RMS each
So even if 2 cones are better you will be seriously underpowering them
Giving 600 Watts to 2000 Watt setup will result in a lot of distortion not even mid way in volume opposed to beatting the out of one 15 at 1400 watts
So your best bet would be 2 12" L5's or L7's bridged to 1 Ohm final load ....

 

actually unless you have that birth sheet for this amp that i got from u the kicker web site says 1320 at one ohm for the first gen of the kx1200.1 also i got a bass **** cable from a JL audio 500/1 and it works 100% thanks again for a super amp

 

i had it somehwere but i lost it ....

 

In this case I would guess more cone would be louder

 

less cone area with the right amount of power will be louder... this will leave you with more/better box options

i went from 2 12's to 2 10's and a smaller batt and gained 0.6db

 

My vote also goes towards more cone area, provided you have enough space for a proper install.
Birthsheets are not representative of the real world. Those numbers are made using a large power supply (read: NO voltage drop) and an constant load, neither of which happens in a car. Sure, it is possible to build an electrical that doesn't drop below 14V, but impedance rise is inevitable.

 

I vote for more power.
Box and placement are extreamly important though. With same power I can make 1 12 louder than 2 15's with the right box and possition.
If you plan on getting another amp in the near future then get the second 15 and have fun. Otherwize you're better off with 1 15 and build the right box.