I think he should probably drop the United States Championship first, although it wouldn't surprise me to see him hold onto it for a while longer and wind up having a feud with Daniel Bryan. That is, if Bryan winds up being the one that wins NXT. Gien the fact that a lot of people are anxious to see a Daniel Bryan/Miz feud, it does seem like a logical way to go. The Miz has been on Bryan's ass from the getgo, the winner of NXT gets a title shot at whomever he wants, it just fits nicely and will give a lot of fans exactly what they want.
Under normal circumstances, I'd be screaming to get the United States Championship off The Miz as he hasn't defended it in a long time. However, the title is still being kept on television week in and week out and having it along with the tag titles has caused The Miz's stock to rise considerably. Think about it, when was the last time you saw an American wrestler in a major wrestling promotion carrying three championship belts? Off the top of my head, I think the last time I saw it was when the late great Dr. Death Steve Williams won the AJPW Triple Crown heavyweight Championship well over a decade ago. Carrying around three titles just plain looks impressive and The Miz is on television every week wrestling, cutting a promo or both so I can't say I've got too many complaints with his run as United States Champion as being the champ, even before he won the tag titles, has improved his standing and stock within the WWE.