It depends on whether you want to use an EF 50 mm 1: 1.8 II or an EF 50 mm 1: 1.8 STM lens with the 6D Mark II or not. It also depends on whether you can do the same with your current APS-C Canon case and the EF 35mm 1: 2 or similar lens.
It probably also depends on what else you want to do photographically, which is not possible, at least in the near future, if you spend the money on a 6D Mark II instead.
Selection of Law or best A camera for a specific task is part of what it means to be a competent photographer. The more knowledge and experience you have, the better you should be able to figure out which of the available tools will do the job. Sometimes you have to settle for a tool that can do it good enough, At other times you may have the option of the best Tool to get the best results for a given project.
To answer the question correctly, Would an upgrade to a full-screen camera with only one lens (for you) be useful?'You need to know what you want to achieve with the EOS 6D Mark II and a 50mm 1: 1/1 lens, and how this camera is suited for the job compared to the tools currently available.
You did not tell us …
- What body and lenses are you currently using for shooting?
- What kind of photos are you doing now?
- Whether your current equipment can produce acceptable results for this type of photo
- What other types of photos do you want to make in the future that your current equipment may or may not allow?
… it is very difficult to answer your question definitely.
For a general overview of the differences between cameras with smaller and larger sensors, see this answer. Is there really a difference between crop and FF sensors in terms of image quality?
Addendum with the following important additions to the question:
I do not have the money to buy both the body and the lens at the same time, but later I would like to get the wide angle EF L 16-35 f / 4.
There is no "right" way to buy two pieces of equipment that have to be spread over time for financial reasons. However, the EF 16-35mm 1: 4 L IS may be more useful for your current APS-C camera than the FF 6D Mark II with only a 50mm lens. And the lens is currently about $ 600 cheaper than the body, at least here in the US.
Both the EF 16-35 mm 1: 4 L IS and the EOS 6D Mark II are excellent value for money. With a few minor details, the 6D Mark II (currently sold to authorized Canon dealers in the US for $ 1,600) is a camera equivalent to the previous generation 5D Mark III debuting at $ 3,400! The EF 16-35 mm 1: 4 L IS is also an outstanding optical performer for a $ 1,000 lens. However, upgrading a camera or lens does not make you a better photographer. It could Allow you to use more of your skills, knowledge and experience.
Equipment with higher skills can certainly help. But a better camera does not make you a better photographer. It will only allow you to use more of the skills, knowledge and experience you have gained while traveling. Part of this experience and knowledge helps to choose the best tool for the job from the options available.
The key to a standout photographer is not the best gear in the hand. You know exactly what equipment you have to know what you are going to do and what you will not do, and then you work within those parameters to get pictures that give you the technical capabilities of the equipment.
Only one main lens is restrictive, but I thought that one buys a FF body
Now I would "commit" to high quality photography, and next year
I would buy this wide-angle lens to really switch to full screen.
You do not need an FF body to create high quality images. You must commit to doing the right things in terms of composition, lighting, technique, and post-processing to produce high-quality images.
I know that FF is not a solution to any image quality problem, but 6D II images are sharper, sharper, and so on. Not to mention photography in low light.
Crisper, sharper, etc. than What, exactly?
You have not told us which camera or lens you are currently using, except one of several possible 1 / 1.8 lenses that Canon has produced over the years. These 50/1.8 lenses are great for the price at which they are sold, but they are not necessarily world-class in terms of large aperture lenses.
"Crisper, sharper pictures" are the result of many things. These include, above all, good light, good lenses and a good technique that uses them.
Another option is to wait another year for the EOS R prize
and then invest in the future-proof RF system
I could then use my EF-S lenses.
No photographic device is "future proof". Everything will eventually be replaced by something better or cheaper. The question that needs to be answered, however, is, "What's available now so you can take the pictures you want to take now?" If you decide to wait until the next, better, new thing is released, never take a single photo because there is always another "new and improved" camera or lens on the horizon.
Note that when using EF-S lenses with a full-screen EOS R, the images are clipped, and you lose the majority of the resolution of 30.4 MP, since only the medium 11.9 MP are used for images, taken with an EF-S lens.