The dual opposed Infinity will outperform the 15" Dayton. While the Dayton is a decent driver the dual opposed have a few things working in their favor:
1. Cabinet vibration cancellation. By have two drivers arranged opposite and in phase they effectively cancel out any transfer function that a single driver would typically transfer to the cabinet. Think about it this way: in Science class the teacher would take a spoked bicycle wheel and spin it at a high rate and then attempt to control it's motion. You would see that person struggle against its momentum. That is transfer function. Now think about have two wheels spinning but in opposite directions (opposed) and then you could control those like they weren't spinning. No energy is transferred because they each cancel out.
2. You get more surface area with two 12" than the 15" Dayton. Roughly 20% more. The Dayton 15" is 829 CM2 the two Infinities combined are 1060 CM2. That's a pretty good improvement.
3. You get better power handling and thermal dissipation with two 12"
4. They model model for around 3 cubic feet enclosure so the 15" doesn't save you any space
5. The Dayton is 8 ohm. Who ever recommended that and the Crown amp either doesn't know what they are talking about or didn't bother to look at the spec. The Dayton would need the XTi 2002 at the least and that amp is $669. The 1002 would work but it's leaving a lot of performance on the table.
Now you could do the XTi 1002 (the DSP is nice) and the Infinity 1260W and wire each dual opposed box for 2Ohm which the Crown will do and handle. It will deliver 700 watts / channel which is line with the RMS on the Infinities. You should probably get more amp than even that (see item 6).
6. The Infinity is probably more conservative on their ratings and it can CERTAINLY handle, one on one vs the Dayton, a great deal more peak power (which leads me to point 7)
7. I would be willing to bet a tear down would reveal the Infinity is a better built driver.
