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.