Its clearly very personal. EMWV (everyone's mileage will vary). I start with what's motivating a desire/need for change? What am I frustrated/happy with when using my existing gear and the results it/me produces? Once in a great while a change is driven by the need to accomplish something outside the capabilities of the existing equipment, although that is extremely rare.
Examples: the move to doing wildlife work for a few years, which needed longer lenses and AF that simply were not available in the current system. Another example is the move from film to digital - that left the 4x5 behind and technology "forced" a new journey, since I did not want to keep up the darkroom, supplies, consumables, etc. I used to print big, then with the advent of digital I started printing smaller and displaying electronically. About a decade ago (2010) I decided to start printing again and printing big (a major solo exhibit drove this decision and it rippled through my entire workflow from capture camera & lenses, to computer, to processing, to wide format printer).
I was very active teaching photography workshops for a decade++ and it became helpful to have gear that was more similar to what my students were using (I was mid-market workshops so Nikon D3s and Canon 1DsII were the high-end of that day).
My short list:
1. What has changed with what I am trying to do?
2. What has changed with technology, electronics or optics - and is it meaningful to my work?
3. Do I believe/care about the system's maker - their ability to continue to fund innovation, support customers and stick around?
4. Where do I assess ergonomics, design and the pleasure of use?
5. Where do I assess image quality?
6. What has changed with me personally physically (vision, strength, stamina, interests)?
7. Who is the dealer and how good are they?
8. Budget or price can not be eliminated as a factor
Item 4 and 5 are paramount, and they both have to excite me. Item 7 is critical, especially with low-volume medium format, you need a dealer that has your back and is big enough in the market to have some vendor pull. I worked with my Palo Alto dealer (RIP) for 35+ years, even when I lived 1,500 miles away for a decade. Now it is Capture Integration

I wish they were closer.
In the distant past I did photography for a living (although always as a parallel career) and that led to an entirely different set of metrics -price/performance, ROI value, depreciation calculus, client demand and rapid service.
I do try to predict how long I think the "new" system will last - although that math is way subjective from my experience for personal work.
Olaf, I know it would likely be unwise for you to declare what you are deciding between, but inquiring minds want to know?
Glenn