Do the bigger side if you under power. Those woofers have progressive suspensions and will handle that well. They move more at lower power, from my experience, but also don't bottom out easily, because the spiders get stiffer as the cone moves further away from its resting center.
A 6" aero might work, might be a little big. That's 28 in^2 roughly. I'd only do a 6" aero if you were going to run the 2-2.1 cubes. That'll be 13-14 in^2 per cube, which is really good for those woofers. The port will be very long, though. You could just buy 6" pvc, the flare with that much port area isn't needed, if you do a 6" diameter round port.