I would suggest 18mm over a 20mm focal length for close city scape laneways and buildings from across the street is the better of the two focal lengths.
I seriously looked into either a 20mm or 18mm focal length lens for close around the city stuff. I managed to borrow an 18mm Nikkor and on on one outing with someone who owned a 20mm Nikkor, I was able to swap and check them out on the spot.
I decided upon the 18mm length as being a far better lens for this type of work, as it made sense when visiting Germany, which I was doing a lot at the time, for photographing tiny streets in tiny villages.
I have 28mm, 24mm and 18mm focal lengths. I consider the 28mm a short normal lens that is sort of wide. The jump to 24mm is quite huge and this is really where wide angle stuff starts; in my opinion.
From 24mm the leap to 20mm is also quite huge, but nowhere as big as the jump from 24mm to 18mm. With 18mm focal length you really are in the world of super wide angle lenses that are affordable in 35mm photography.
If I was going to shoot narrow streetscapes, then the 18mm is the lens I choose. If I'm looking for the ability for some wider streets, then 24mm is maybe the lens, but you can always walk towards your subject (mostly) and use the 18mm.
Are you aware of aftermarket wide angle lenses? For my 24mm and 18mm wide angle Nikon mount lenses, I am using Sigma lenses. The Sigma lenses which are AiS mounts, were very good, with the 18mm being quite good. I compared my Sigma f/2.8 18mm lens side by side with the Nikkor f/3.5 18mm lens with two F3 bodies on tripods. The result is that the Nikkor has slightly more visible contrast in the viewfinder and obviously produces slightly more contrasty negatives; but the differences overall weren't that great. At f/5.6 there isn't much, if anything, between them, except for the aforementioned slight contrast difference
I bought my Sigma for $250 secondhand in the early 1990's, it has a 72mm filter ring. It has floating elements, which is similar to the Nikon CRC (Close Range Correction). This allows very close focusing to be achieved; I have done some landscape photography with this lens with the closest in focus object being 300mm from the lens, sharp as a tack. At the time the Nikkor was somewhere around $1450.
If it matters, my Sigma doesn't have the prong at f/5.6 on the lens.
This picture was taken with my 18mm Sigma lens. That is the full frame, I was standing very close to the church.
Had lunch on the lee side, out of the wind, decided after eating, to check out the possibilities and liked what I saw. Waited a while for the clouds to be in the right place. It's an interesting building as it's been built on the cheap. The big...
Ultra-wide angle prime lens. Announced: April 1984. Production status: Discontinued. Maximum format: 35mm full frame. Mount: Canon FD, Contax/Yashica, Fujica X, M42, Minolta SR, Nikon F, Olympus OM, Pentax K, Rollei QBM. Diaphragm type: Automatic. Extreme angle of view. Fast speed. Floating...