I have had the 28/3.5 and the 28/1.9 since they came out. The 28/3.5 is one of the best black/white lenses i have ever used! There is a quality to the images that is different from the f1.9 nokton and the Z1 28/2,8 and the leica 28's.
If you do a lot of low-light shooting the speed can be a problem, but the R4 is smooth enough that you can handhold down to 1/15 with little or no problem. The Nokton 28/1,9 is bigger and i find myself using it less and less. It is a match for the Summicron 28, except when wide open when the corners appear slightly soft. On the R4 you will find that the 28/1,9 does show up in the finder, whilst the 28/3,5 is virtually invisible there1
To put it in perspective, I have 1 28/1,9, 1 ZI 28/2,8, 1 Elmarit 28/2,8 (from 1980 and still a remarkably good lens) and three (3) 28/3,5! One is black, one is chrome and one is in Nikon Rf mount!.
The difference between a 2.8 and a 3,5 in size makes the 3.5 very attractive and in most cases the 1/2 stop speed will not be a problem. The step from a 3.5 to a f1.9 ( let's face it, it is most likely a f2!) does give you some latitude with speeds, but it is a lens big enough to attract attention when you shoot in the streets.
The 28/3,5 looks too small to be "significant" and most bystanders ignore it! Same thing with the 21/2,8's from ZI and Leica. The 21/4VC attracts little attention from the surrounding people and just like the 28/3,5 " packs a punch".