Duke,
How does the Hathor 212 compare to your Fantastic 1505?
The two have similar maximum sound pressure levels, but the 212 takes twice as much power to get there. In return, you get a more solid low end, deeper & more impact, with the 212 (two motors instead of just one = more impact). The mids on the 1505 are more aggressive in "growly" mode, and there is a greater difference in voicings between the two midrange settings with the 1505. The 212 goes higher with its pair of 3" mids.
A couple of weeks ago I showed the Hathor 1533 at a GTG in the Dallas area. I mention it here because it figures into the same general ballpark as the two cabs you asked me to compare. The 1533 combines the top end of the 212 (which is of course quite similar to the 1203) with the more growly upper mids of my custom 15" woofer. So in some ways the 1533 sits "in between" the 212 and 1505 as far as voicing goes. Low end of the 1533 is identical to the 1505, but because the more beefy 5" mid allows more overlappage in the crossover region, the 1505 is a bit more aggressive than the 1533 in "growly" mode.
People who own the 1203 have told me that the 1505 (or 1533) has a bigger bottom end. It does seem to sound that way, but actually the 15" Hathor cabs are -4 dB at 64 Hz, while the little 1203 is -3 dB at 64 Hz. The shape of the low-end response curve of the 15" cabs probably averages a bit higher relative to the rest of the spectrum, and my guess is that that accounts for the perception of "big low end" on the 15" cabs.
The 1505 and 1533 have the "4/8 ohm" switch, so they can be set to "8 ohm" mode (at the loss of about 1 dB of efficiency) and safely paralleled with another 8-ohm cab if needed. If you don't need that feature, the switch can be omitted, saving you about a pound in weight.
If there is one area in particular that you'd like for me to go into more detail about, let me know.