Some guys run guard rings and some run full covers. I like the full covers bacuse they are beefy. Full covers usually cost more $$$ but when Solid puts them on sale they cheaper then rings. When you see them on a Jeep you really can't get a feel for how thick full covers really are. Also Soild will send you new sealant, new bolts, a lifetime warranty and a Sticker.
If you can afford it, I recommend front and rear covers. My rear one has been hit as often at the front, but mostly from sliding the rear diff over something.