Heya,
I choose Canon, for starters, because I'm used to the system and I like their lens line up and the third party lens options, as well as the ease of adapting vintage lenses. Canon consistently has the edge on autofocus systems and their ISO seems to be great for performance, even if the sensor tech is not the industry leader.
From there, I stay with SLR because of the interchangeability of lenses. Now that mirrorless is here and there's interchangeable lenses there too, the reason to stay with SLR is starting to grey out between the two. I will keep both systems for now, as I have SLR and Mirrorless at this point. But Mirrorless doesn't have the robust autofocus that the SLR has yet, so until that changes, and the Mirrorless has speed and autofocus that can match even an entry SLR that is modern, then I will continue to keep both.
For me the point of having this system is to have the ability to have the right tool for the job, and that's the lens mainly. Different combinations of focal lengths and apertures, and optical qualities such as sharpness at those apertures, contrast, color, etc. There's no reason in my mind to stay with SLR if I was going to just use a single lens of any kind, prime or zoom. It defeats a lot of what the SLR provides. There are plenty of good quality cameras that have a single permanent lens that is either like a prime, or a zoom (and sometimes a super zoom). When I think of going down to 3~5 lenses, I just feel like... well, why bother keeping the system at that point if another system can do all of that with one? But then I look and see... no one system covers it all, so we come back to the property of these cameras that is interchangeable lenses. This lets you use the right tool. If you're compromising it's by choice, or by limitation of having it.
I have 4 cameras currently, 1 Mirrorless APS-C, 1 Full Frame SLR, 2 APS-C SLR. They all share the same lenses so I can use the right tool for the right job. But frankly, the camera matters less and less, and the more you use several cameras, the more you find that it truly is just a few features per camera that make a difference for what you're doing, and it's subjective and preferential.
I have a bunch of lenses. I buy & sell often. I've bought the same lens twice. Several times now. I get it when I want it for the job, and later if I'm not using it, I get rid of it again. They hold value. The cameras hold no value. But the lenses seem to hold very well over the years.
I use both zooms & primes. I vastly prefer primes, but zooms are more affordable than primes for certain things. And that's due to physical size of glass (no one is buying narrow aperture telephoto primes; or even wide primes with narrow aperture; we buy primes for their wide aperture that zooms simply don't do).
My lenses range from 11mm to 600mm at this point. Zooms & primes. I have quite a few lenses that are the same focal length (28mm, 50mm and 85mm) but have different properties (aperture, bokeh character, etc). They range from modern to vintage (lots of Pentax M42 mounts).
I prefer to have 2 cameras on me, one with a prime, and one with a zoom (or another prime). I do a lot of manual focus photography too so which body I have dictates if I'm doing that or not.
The day I no longer use primes, will be the day an affordable zoom comes out that has F1.4 or F2 constant, and it's prime-sharp at those apertures. Sigma is inching closer and closer to this being possible. So it's entirely possible. But I also appreciate the weight and size of a prime. Carrying a 7lb brick in your hands, or two, gets old.
What I shoot varies. Like tidal waves, sometime I'm shooting a lot of one thing. Then I change it up and I shoot a lot of something else. I shoot primarily macro, landscape, portrait, astro and wildlife. I shoot mostly wildlife, portrait & landscape weekly. I used to shoot macro daily when it was cooler outside. So I have a lot of tools for each.
Lately I've been shooting a lot of 11mm F2.8, 22mm F2, 85mm F1.4 and 600mm F8.
Regardless, I vastly prefer having several cameras and several lenses than one setup with one lens. I can do more with different options that I all want, with no down time.
I'm about to head to the local swamp, and I will have a gripped APS-C with 150-600mm VC zoom attached in my hands, and on a strap, my EOS-M with 22mm F2 in tow. Later tonight, I will probably have my 11-16mm F2.8 utlrawide on an APS-C, and again, a 22mm F2 prime on another camera or possibly a 35mm F2 IS on it, if tonight's sunset is worth going out for. Who knows. But I change up my range from 11mm to 600mm in a single day in two different locations hours apart often. I shoot near daily when I'm not at work.
+++++++++++++++
In an ideal world, I'd want several options, and these are along those lines of what I would want to cover everything I do on a weekly basis, so it's not just "once in a while."
2~3 cameras. I want a full frame (6D would be fine; I currently use a 5D), an APS-C (any modern one would work, 70D probably ideally right now; currently use an XSi & T4i/650D) and a mirrorless compact (EOS-M serves this fine for my use of it now).
11-16 F2.8 II ultrawide. It works on both full frame & APS-C. Ideal for me because it's fast for night use in astro, and with a multi-coated filter, I don't get bad flares anymore, so it functions in the day too perfectly fine.
22mm F2 for my EOS-M, pancake. Travel, walk around, day to day when I don't want a bulky camera and big lens around. Performs in low light like restaurants & bars.
35mm F2 IS for portrait with context and landscape/astro/etc/it's a general walk around lens for me often.
85mm F1.4, for portrait work; 85mm F1.8 for sports and indoor events where speed maters.
180mm macro prime. 2.0X TC involved.
600mm prime or zoom, I would prefer a prime, but they're too costly at this range.
Tons of vintage primes that I use for fun. Like a dozen.
Very best,