Should I start with Canon or Nikon?

I'm going to be buying my first camera pretty soon here, and I need to decide if I want to buy Canon or Nikon. I think I'm more of a stills guy, which is why I'm leaning towards Nikon (because of their better sensors), but video is something I want to try and could maybe get really into, so I'd want to go with Canon (because of their better video features like dual pixel AF) if that happened. I want to figure out what brand to get now because from what I've heard switching is a difficult and expensive decision.

Added (1). Another reason I'm leaning toward Nikon is the easier "upgrade system." Nikon has clear paths you can take. For me, I plan to get a D3400, then a D7500, then a D750, then a D850 (photography is just a hobby right now, I don't need a D5). With Canon I feel lost.

Nikon ayeee

I prefer Nikon but it's me. Try them both and decide.

For video go Sony. They also make great photos, as do Pentax or Olympus.
Switching systems is always expensive as you can't use your old lenses and other accessoiries anymore. OK tripods excluded.
It seems you have decided on a path through the Nikon world what I don't understand why you have so many steps planned between the D3400 and the D850. Why not get the D850 right away if you have the budget. Being a beginner does not mean you have to start with a cheap camera. The better ones are often even more user friendly having better autofocus and automatics and maybe a wider ISO range and you have no excuses why the pictures do not turn out. You can't say "it's a cheap camera".
Anyway if I say Canon you would not go for it. I think you're only hunting for confirmation of you choice out of just two brands.

Your upgrade plan makes (vague) sense if you are planning to upgrade every 3 months. If you want to spend a year getting familiar with the 3400 and another couple of years using it to find what it doesn't do that makes you want to upgrade then there will be a 7700 that is a bit better than the 7600 that was a bit better than the 7500, and another 2 years down the line after that there could well be a 770 that is an improved 760 that was an improved 750… Etc. Technology moves on and you have no idea what cameras will do in 5 years time.

All DSLRs do exactly the same, have the same PATMB controls, the same exposure compensation and in some cases the same pixel count. Differences are in fastest shutter speed, frames per sec, buffer sizes, autofocus capabilities, focus points, cross focus points, ISO range, CF and/or SD card storage, Wi-Fi, video capabilties, crop factor, a few other inbuilt bits and pieces (timelapse, HDR), tweaks and controls and lens range.

Do not get obsessed with the fine differences between the cameras and what they can do. Unless you are particularly choosy and counting pixels you can take pretty much the same photo with every camera, and out in the real world you are going to find that your ability to use the camera and getting the lighting right will have a much bigger effect on the image you create than anything the camera does, from whatever manufacturer. And, ultimately, unless you buy the good, and expensive, lenses, the camera body will have little effect on the final image unless you are going to extremes of lighting (high dynamic range or long exposures where sensor performance can be more important).

There are also Pentax and Sony who also make excellent cameras that are better in some respects than Nikon or Canon, but don't have the wide selection of lenses that N & C have.

Get the consumer camera, of whatever manufacturer, decide what photos you want to take and develop your skills. A camera and the lens are tools to do a job and if it's not working out, a camera upgrade won't give you better pictures.

Not sure why Canon are confusing, their website breaks the cameras down into beginner, enthusiast and professional.

As for video, sure you can use a DSLR, but you may need a bunch of extra kit, costing many £$, to make it viable. You have to use the rear screen to see the picture which can make it awkward to hold the camera and is also a pain on a sunny day when you may not be able to see the screen properly in the first place.

For the record I use a Canon 6D and 7Dii.

Either will work, and other brands out there work well too. Shooting video with a DSLR is a hassle compared to a good camcorder. The only thing that is notable in the Nikon lineup is the 200-500 lens which is very popular with the birders.

If you have friends that shoot a particular brand, I would go with that as you can try out their accessories and geek-out over the kit.

Nikon

This is why I refer to, "The idiots' Two."

If you want stills, Pentax (the oldest SLR manufacturer in Japan) use the same (Sony) sensors as Nikon. You also get the ability to meter with legacy lenses and in-body stabilisation, with ALL models, and pentaprism (brighter) viewfinders and weather-sealing at a much lower cost. Canon and Nikon spend large amounts of money telling anyone who'll listen (and a few who won't) that they make the best cameras in the world. Pentax just make great cameras.

There are many questions you SHOULD ask when contemplating your first DSLR, but this is not one of them.

I started with Pentax (for film).

They do great with digital, too.

One advantage they have over all others is that their image stabilization is in the camera body,
not in the lens, so you get that function no matter what lens you are using.

That's the way I would go…

Yes if you intend to persevere and go a long way in photography. Either brand matches the other one all the way, so it's just down to personal preferences relating to things like handling and connectivity. Only if you want to photograph black cats in a dark coal cellar will a Nikon just pip a Canon to the post. So there should be no need whatsoever to jump ship to another brand, unless it be the Sony Alpha 7 series with an adapter for your chosen mount.
Don't go on an incremental upgrade path. If budget permits then go direct to 35mm sensor. Otherwise get an 80D or D**** which have articulating LCD screens.

Whoever says that this brand is better for this kind of shot and this brand is better for this other kind of shot does not know anything about photography. Then there are those people who claim to be photography experts but know only two camera brands.

If you're not loyal to a brand yet, choose wisely. The best way to go about it is to test the cameras yourself at the store. Ask the salesman to set the camera to Auto everything and just take pictures with it. Just make sure to compare cameras within the same price range. One final note though, the dSLR was never meant to do video.