if you push the subs with enough power, the plywood will flex. MDF is a more rigid material. plywood may not be airtight due to natural defects in the wood. MDF will be airtight. just remember to glue, screw, and caulk the seams.
I'LL TELL YOU I'VE BUILT MANY BOXES IN MY DAY AND THE BEST ONE I'VE MADE SO FAR I USED 5/8" PARTICLE BOARD. I THINK 5/8" PARTICLE BOARD IS THE WAY TO GO MAN.