I am interested in street photography. So I'm looking for something portable and I wonder if there is a difference in relative weight between the two objectives.
It's really a matter of preference. On occasion, I hike with a Pentax 645N. At the time, shooters used things like a Graflex. So, if something is "portable" or not, it's not something that someone can decide for you.
As for the differences in the lens, the 17mm is 120g while the zoom (if I look good) is 91g. A difference of 29 g equals about 1 ounce, or a single slice of bread. If it's the drip that makes yours overflow … well, remove something from your bag and worry less about it.
I also like doing black and white photos and I sometimes need to zoom in and cut them later. I am therefore looking for a solution that allows me to preserve the sharpness as much as possible.
"Make black and white" and "zoom and cut" (also called "crop") have nothing to do with each other. All digital images will become unwanted if you trim and twiddle back at one time. The only thing you can really do is crop and not interpolate but limit yourself to the resolution you have now. In today's high-resolution sensors, this is generally enough, but you have to wonder if the lens is able to solve these small details. This varies with each camera, lens and camera + lens combo.
Again – this is something not to worry about. If you reduce your shots too much, you need to buy a longer lens. Period.
Another requirement is to be able to focus quickly, because I capture moving subjects: I wonder if the choice of lenses makes the difference.
There is an old street shooter who says, "F / 8 and be here." The shooters fixed the lens at a preconceived distance, the camera at f / 8 and the depth of field occupied the rest. Sometimes the hyperfocal distance was used. It just allows you to aim and take a picture.
Even with fast autofocus systems, this method remains very valuable. Why do I report it? Just to show you that the quick focus is as much an operator skill as a camera feature.
Finally, is there a significant difference in their market value?
Of course there is. The premium seems to be about $ 150 more than the zoom. Why is it important? The market value does not affect the images taken. One of the most used lenses in the world is the Canon 50mm f / 1.8 – and it's brand new at $ 125.
What is the best option between the two lenses?
Simple really: whatever you choose.
Your questions indicate that you are very new in this game. I can understand the pressure of a first lens and to be honest, there are many advantages and disadvantages for one or the other of these. choice.
You will divide the photographic community in half on a choice between a zoom and a premium and you will hear both opinions. The best thing to do is to try them yourself. Renting the camera and both lenses is the best option to take pictures with both, see and feel the differences, then make the best choice for you.