Despite being British and having ridden Nemesis vastly more (I've done Nemesis 29 times, with my first ride being in September 2013 and my most recent being in August 2022, while I've only had 1 ride on Montu that was back in August 2016), I'd actually be inclined to vote Montu here.
I do like Nemesis a lot, but I've always found it to be ever so slightly overhyped, for lack of a better term. Don't get me wrong, it has brilliant pacing and intensity and some absolutely brilliant elements, as well as a very unique layout design. It's an absolutely excellent coaster, I like it a lot, and I can't wait to ride it again in a week's time! However, I've never quite subscribed to it being placed on the absolute godly pedestal that so many British enthusiasts in particular have it placed upon. Controversially, it's not even my favourite coaster at Alton Towers, and I even dare say that I might prefer Nemesis Inferno at Thorpe Park as a track and train ride experience... although I can never quite decide on a victor in the Nemesis vs Inferno comparison. One day, Inferno will come out on top, whereas Nemesis will come out on top on another day; they're very closely matched in my rankings!
By comparison, I remember Montu being absolutely amazing, and still my favourite B&M Inverted Coaster! The intensity and sense of speed were amazing, and the raw size, length and pacing of it just put it a cut above the rest, from memory! Although as I say, that was based on one ride, and one ride that was now nearly 8 years ago at that...