They both have their place. The round definitely has more strength than the slot, but slots make for quicker lacing jobs (if you have a 3-4 slot chisel). if your braid requires that you go back through the hole like a mexican round braid then a round hole is the only way to go. For simple cordovan braids I prefer slots because it gives you a cleaner edge. In any case if the joint is under stress you should go with a punched hole. You can close the gaps left in the leather from hole punches by lightly wetting the edge after lacing, this will tighten up the seam and make for a nicer edge. For a bible cover I imagine you could get away with slot, depending of course on the braid you're doing.