As far as your concern about the SIM card, I think that they are both in the volume tray. So, you're going to get a nano-SIM there either case.
The biggest differences are going to be the battery and customizability. If you forgot to charge your Maxx overnight, no sweat. But if you want a customized phone that looks cool (I hear the wait is a little less than a month away), go with the Moto X.
You're getting the same basic phone so you can't go wrong either way.
Edit: My personal advice, see which one goes on sale. It could be that once Verizon gets MotoMaker, they could have a sale on it. Or maybe BestBuy or Amazon has a great price on one or the other.
Time flies like an arrow, fruit flies like a banana. - Groucho Marx
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