So between the two, which would have the most potential for accuracy? The Colt 6920 at the prices they are nowadays is pretty tempting, and I'm sure they are durable. But with the new SW 1/8 twist and 5r rifling, with mid length gas, they seem like a pretty solid buy as well. Thoughts?

Chances are that any good nitride barrel, even if not 5r, will have better accuracy potential than rack grade chrome lined Colt barrels.  Nitride treatment hardens the steel, itself, at the molecular level, wiyhout changing the dimensions of contours and edges of the rifling.  Chrome lining is a plating, which can be uneven.

S&W's wholly owned subsidiary, Thompson Center, makes those barrel, and TC has a good rep for accuracy.

The M&P’s look sweet. I don’t know why they aren’t more popular here. I like the Magpul MOE mid-length version.

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Honestly... On paper the M&P has better accuracy potential. In the real world both rifles are 1.5 to 1 MOA with the right ammo ect. Both are great rifles and well made. The Colt has the mil spec and battle proven track record but the M&P has proven to be very high quality at low cost. Choice is yours so pick the rifle you like best you will be happy with either one.