I just finished shooting for two month with Nikon Rf's. I do that occasionally - just to avoid being too Leica centric.
Biggest difference is not really the viewfinder - they are all good to excellent. It is the precision of the focus. The long "throw" of the Nikon helicoil makes it easier to pinpoint focus. It is a bit slower initially - but when you learn the trick of "pre-setting " the focus as you lift the camera to the eye - it works well.
A major difference is the quality of the early Nikkor lenses - such as the 50f1.4. I have multiples of these, ranging from early 1950 (#5005xxx) to the Millennium version. Apart from some contrast increase in the later ones - they are as good as you will ever need. The 35's hold up well, even compared to late Summicron's, particularly the 35f2.5 - which is much cheaper than the 35f1.8 too.
Bodyshape on the Nikon is a bit akward - sharp edges and "boxy" - again, you get used to it. The focussing patch is not as sharply defined as the M's "box" - but again, it is a matter of experience.
Of course, it focussed "backwards" from the M and initially you spend a lot of time correcting this. I find that using M's and Nikon's at the same time is fraught with disaster - plenty of artsy, fuzzy, shots!
In short. they are different, but similar!
One added advantage is that Nikon's seem to survive better than M's. Yesterday I handled a mint S2 with 50f1.4 (early chrome dial - probably late 50's). It had obviously been sitting for at least a decade or two - not used. I carefully wound it on, checked the speeds and apart from a slightly optimistic 1 sec - they were all dead-on. I also handled a M2 (1957) - again, mint and had sat in a nice, dust free cabinet for the last 20 years. Very stiff shutter, all speeds were off, shutter was dragging badly. I would not had a problem loading a roll in the S2 and shooting it and get correctly exposed shots. The M2 would need a $100-120 clean and lube.
As a first Nikon Rf - either go for the S2 and a 50f1.4 or a S3 with the additional 35/105 frames - either one will have you drool over negatives!!!!!
If you check our Flickr - and tag "Nikon Rf Month" there are about 700 shots in bl/w done with Rf lenses, from 21f4 to 135f3.5 and bodies from Nikon S to late SP's.