i would always go for more surface area... that is with optimal power dont get a bunch of subs that u cant power... but sealed sound is better sq IMO ( i say that cuz sombody might argue with me ) but ya 4 sealed gets my vote
Install dependent. You could try to shove those two 15's into a horribly small airspace, and since we don't know how much room you're working with, nobody knows for sure if you have enough airspace to properly house each setup.
Personally Id go for the 12s since theyd likely be very close in SPL to the 15s, but the sealed box would sound better.
15s would do well in a 5 cubic foot box tuned to 28hz (seperate 2.5ft^3 chambers)
12s would do well in a 4.6 cubic foot sealed box (seperate 1.15ft^3 chambers) but can go into as little as .8ft^3 (smallest recommend box size is .2ft^3 lol...I didnt know they went that small)
when you say 500 watts is that per sub or total? if all you have is 6 cubes to work with not sure if you'll fit either setup because the box will be bigger externallay when you factor in wood sub or port displacement. personally i liked to run my rl-p's at 1.5 cubes each.