4th order or rear loaded horn????

been out of car audio for about 2 months now, but the rest of the year is looking promising... i have everything i need for a complete system just wanting to recone the ns 12's i have to 15s and i have been going back and forth with pete about a rear loading horn for some time now but cant make my desicion...

simple question is that what would the advantages and disadvantages of either set up.... power is 2 D2 ns, 2 1500ds @ 2 ohms strapped, 2 dekas 9a31s, and a ohio gen 225/160 HO that has not been dropped in yet... previous systems were 1 15 fully loaded btl in a chambered t line.... which was the loudest set up to date, and 2 ns 12s in 5 cubes @ 35 (which was way too big... hence the new plans to recone one of from burning up) but from each set up i have learned a ton of info in the short time i have been experimenting with car audio.

i was told by pete that the horn may give me the range i am looking for and sound good doing which is what i am looking for and wanting to ask him the advantages but he's is impossible to reach in a timely manner and this forum is pretty good in response time and knowledge, any input is awesome.