I can't remember where but I saw some tests that showed an un-dressed figure 8 had a higher breaking strength than one where the ropes crossed. (it's kinda academic given the sort of forces required though)
Personally I use a bowline + stopper for climbing, figure 8s are a swine to undo with pumped arms if you fall on them hard & if you're pulling the rope through gear there's always the worry you won't fully untie knot.