Welcome to CPF where the stock answer is "buy both" as they meet slightly different needs <smile>.
The lights are relatively equal in my opinion.
The bigger question is rechargeable vs. disposable batteries.
Also accessories vs. just a light.
The M3T with the stock MN15 lamp will be a little less bright than the US but will cover a larger area. It will run for an hour off 3 123s (more or less).
With the 20-minute turbo lamp that I think comes with it, (the MN16) you'll definitely light up a larger area than the US and it will be as bright.
With the 1-hour MN2 lamp, you'll have a hair less light than the MN15, but it will throw as far or farther--and still cover a circle as big or bigger than the US.
The US will focus to a wider circle while the M3T is fixed focus.
Replacement lamps for the US are a lot less expensive.
I prefer disposable batteries, but if you're using it every day, rechargeables make sense.
With that said, in my L4 and KL1 (on E2e) I do use PILA rechargeables, but keep disposable batteries as in-use backups should the rechargeables run down.
Both are fine lights. I think the M3T is a bit shorter than the US and lighter.
I don't really have an M3T--I have an L6 with a KT-4 accessory turbohead--which is really the same thing.
I have posted numbers in
this thread
Oh, and I gave a good friend of mine an US as a housewarming present. It was an amazing house and they needed it to see around their property <smile>.
Cheers,
Richard