I have the Canon 600D with the 10-18mm f4.5-5.6, 18-135mm f3.5-5.6 and 50mm f1.8. Now I am looking for my next lens which I do not want to spend more then 1000$.
I was thinking of 70-200 f/4, but I am not really often using my 135mm at my zoom lens. Next I thought about upgrading the 18-135 to 24-105 f4. But I often read that image quality is not really better, so why spending the money here?
I am mostly shooting landscapes, nature or city. So actually travel. Do you have recommendations?
Yes. Stop shopping; start shooting.
The lenses you have are what most folks would already choose for landscapes, cityscapes, and street shooting.
If you don't know what lens you should "upgrade" to, then chances are good, you're not ready to upgrade. You need more experience with the gear you do have. And it's when a specific frustration starts to eat at you with a specific thing about a specific piece of gear, that it will become clear you may need to spend money on something to solve that frustration. But you have to know your baseline technique is good, and that you've already tried your current gear to its limits before you'll know you need something.
Want something is different. Everybody wants newer shinier gear for no very good reason other than that it's shiny and new. Some of us can afford to indulge this whim. Most of us cannot. So, it's probably smarter to wait until we know what it is we need.
From Aaron Johnson's What the Duck 174:
In addition to this, don't fixate on just the camera bodies and the lenses with your spending. Gear-wise, support gear, lighting gear, and post-processing software are possibly more effective ways to spend cash; and books, classes, workshops, videos, or airline tickets may actually be what you really need to improve your photos.
See also: Lens upgrade paths (sub $1000) for the EF-S 18-55mm IS kit lens for Canon APS-C cameras
Choosing the "next lens" you need should be based upon a particular need that your current lenses are not capable of meeting. So in order to answer the question you must first ask yourself, "What kind of shots do I want to take that I am not able to take now?" Only then can you answer the question, "What lens will allow me to take those shots?"
Landscape and architecture involve (mainly) static objects, this means that you can use a high focal length lens with a small field of view to take pictures and then stitch those pictures together to compile an extremely high resolution picture of the desired object. A telephoto prime lens, like e.g. this one will then be the ideal sort of lens. You then need to carefully check all such lenses on offer for the sharpness and convert the sharpness measure in terms of lines per image height into the angular resolution. So, a 100 mm lens has to be more than twice as sharp as a 200 mm lens before the 100 mm lens can be judged to be better (but here you must consider the resolution limit due to the finite resolution of your image sensor). Other issues like the sharpness at the edges must also be considered.
This approach has the drawback that it requires quite some work to get to the final result, but it has the advantage of yielding much better image quality. In the event that the resolution is "too high" you can always resize the image, which implies an averaging over pixels so it yields a lower noise image than one taken with a lower focal length lens.
If you don't want to go this route and your focus is mainly on taking single snapshot landscape pictures, then you are better of with a good zoom lens.
