Displacement is king, always. So the only issue is whether or not you have enough mid-bass to level match the subs with the front. You do have near field going for you and if you keep the back seat up that lowers the total output from the sub volume as well.
Typically a 5.25" is better with mid-range and a 6.5" will start beaming sooner in the midrange but provide better mid-bass and lower Fs. It's really speaker specific though