When wheeling rocks going forwards, I place tires to keep the diff from dragging and banging.
Only time I typically back up when in the rocks is when I'm in trouble.
When I'm in trouble, stuff gets used.
I am not as concerned at placing wheels to protect the diff when i am in trouble as I am at getting out of trouble.
So, for me;
Rear is more likely to get damaged, and if the rear is getting damaged there is a good chance the front will as well.

 

Rear for me. Peeled the factory cover multiple times. Installed an AEV diff skid rear and Poison Spyder covers front and rear. No worries now.

 

My friend popped the bottom bolt on the rear cover, when it pealed back. Had to send another Jeep out for oil, while we got the broken bolt out. Banged the cover back into shape. Really glad he had lubelocker gasket on it.

 

I peeled the cover back my stock cover already the third time out with this jeep. Hammer and rtv reseal fixed it but I will be replacing them with solids in the future.

I wouldn't say you could pick one over the other. Just replace them both and call it good imo.