At 650 Hz, both TD10 or TD12 are nearly omnidirectional, since the wavelength at 650 Hz is about 20 inches long, which is nearly twice the size of both woofers diameter. I wouldn't worry with either choice.
If you choose your impedance well, two 10s will take the same power and play roughly as loud as a single 12 of the same model (they will have similar efficiency/sensivity).
If your horn is really 105 dB/W/m, then you would need quad TD15M... a bit overkill if you ask me. Better to go active with separate amplifiers... especially because each TD15M can tolerate over 500W RMS, and your compression driver will blow up well before you reach that number.