It appears (and it makes sense looking at the numbers) that the DPL 15 would be the way to go. The MJ has it edged on max SPL up high, but the 15 has more output down low and a much more manageable response curve.
You can't go wrong with the DPL-15, it models extremely well in a IB. I did a comparison between the two and the DPL-15 has 3db more output at 20hz with only half as much power input as the MJ-18M. For the price, the DPL-15 is hard to beat.
I know that you have most likely already made your purchase but this is more to anyone who is reading this and researching prior to their own purchase.
When looking at 18" vs 15" it is very easy to get tied up with size. When looking at a build for an IB application I myself looked at cost but more so looked at how much room I had to build my IB space and how much air I could move with 18" vs 15" speakers. When I did the math and research I found that for the cost vs space needed I could use 4 - 15" more efficiently than 2 - 18". Then I looked at overall SPL and made the choice to go ahead and spend the extra money and do my build with 8 of the DPL15's and a Face 1200 amp. That way I have the best of both worlds. I have the added sensitivity of going with the extra drivers with a reduced effort on the drivers to give me good SPL if needed.
Yeah, I know, it was overkill but Kevin was great to work with.