If you want to do ebay there are much better. Even if its a lower power amp than what the pyramid says itll probalby actually do more. You don't have to spend thousands if you know what to look for and what is good quality. Some people think spending thousands on thier engine a wast. Its all realtive.

As for the origonal question it should be easy to swap them out and see which one you like better once you have the wiring in. The 12's will probably be more efficient due to more cone area but if the speaker effecience is less it may be pretty close. box size will also play a roll. Personally id run the single 15. I don't think 75 to a sub is enough.