As a huge Fritz Lang fan, I strongly disagree.
M is a darling of mainstream film critics, but in my opinion it's absolutely atrocious, massively overrated, and not worthy of shining Metropolis' boots.
That said, Lang has some other masterpieces I would recommend, like Kremhild's Revenge and the Dr Mabuse series. I'd watch those long before touching M.
Back to those German reviews of Metropolis, I wouldn't put a lot of stock in to proto-Nazi Germany's discounting "sentimentality" in Metropolis. We know where hard-hearted disavowals of sentimentality led.
I get why your assumption they disregarded sentiments exists; we are living in very similar times where undue public importance is given to sentiments. And we would like to think they were completely different to us, but the reality is we live in similarly sentimental times.
Edit: looked up on the author of that review, https://en.wikipedia.org/wiki/Herbert_Ihering, apparently liberal-left leaning and not a darling of the Nazis.
PS. if you want a trip into weirdness, watch the uncut version of Metropolis. Weird mystic religion they got over there.
But also, as ideology, they did looked down on huge range of sentiments. Feelings related to empathy, sadness, actual romance anything like that was seen as weakness and feminine. They were not ok with sentimentality as we normally mean the word. Sentimental movies were not what they approved, because that makes you feel empathetic and gentle and that was bad.
They were ok with only some emotions, notably the ones that makes you more likely to join the fight. Like, relatedly, they were homophobic a lot, but Triumph of Will has clear aspects of homoeroticism.
(For the record, it is not nazi only thing. Any movement that seeks military domination and genocide is going to have similar preferences. Emphaty does not go well with the plans, irrational loyalty to state/party/whatever does. )
A fairly good transfer is available on Netflix DVD.
I think this is being way too harsh on the critics. Sadly, I forgot to add the sources above. The first quote is from a liberal newspaper, the second is from a satirical magazine (Simplicissimus [0], certainly above suspicion when it cames to Nazism) and the last one is from a communist newspaper. H. G. Wells himself notices a "sauce of sentimentality".
> We know where hard-hearted disavowals of sentimentality led.
I don't want to start the discussion whether Metropolis is a pro-Nazi film (it isn't), but we certainly know where a hard-hearted embrace of kitsch sentimentality led.
No, I won't name one as it would ironically dilute my point.
[0] https://www.youtube.com/watch?v=d_I2akJ0k2w