Dozer--- I did exactly as you were thinking; picked up a 508a a few months ago, and got the 716c on close out. Also have the 503t but that's for another day to discuss.
I have not been disappointed with the equipment- both are drilled in the 50/60 by 4.5/5.5" by 20/35 range (Similar but not exactly the same; I wanted them to compliment each other well)
The 716c is about 3-4 feet earlier than the 508a, and overall about 4 boards stronger. The only downside to the C is when they start to break down, I lose carry when moving inside to open up the lane more. The 508a plays very well on fresh with extremely clean backends, and then again after 4 games on the pair (broken down mids, backends starting to recover from carry down).
For the price right now, drill them up. I'm pretty impressed with their 3-800 series balls. Had a 912t but got rid of that when I dropped to 15lbs.