i recall a thread a while ago, mouthguards. awaiting my custom order right now, but basically what the thread stated was that uppers are primarily used due to the fact that the lower jaw offers a very wide range of motion from the skull as opposed to the upper jaw, which is fused into the skull.
i guess what it means is that you have a better chance of busting your upper teeth from your head than the lower teeth.
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some interesting facts as well. but yeah, go with a custom no matter what. check the reviews sections at the top of these threads for more info.