"What Scott said."
If you can swing it, go with the RL-P18. It can move/sweep 6.5 liters of air, compared to about 4 liters for the RL-P15. Here is the transfer function for the two subwoofer alignments as you described:
Above 15 Hz the 18" driver walks away from the 15" handily. At 15Hz the RL-P15 is about done and the RL-18 is dropping off too due to the 16.5 Hz tune (and box size). But one potentially serious problem I see with the 15" plan is the port air velocity. If you feed it 500W, your port air velocity hits 34 m/s which is twice the ideal maximum of 17 m/s (5% the speed of sound).
