I'll be the pro-Mac guy on the subject....
First off, in the Mac vs. Windows thing, you should get what you like best. They both have pros and cons but ultimately if you prefer one or are used to using one, that's probably the best. You can get suitable hardware in either.
How much power you need really depends on how much you're going to be doing with it. You can get a woefully underpowered PC just as easily as a Mac system. Yes, the Mac system will tend to cost more up front, but that doesn't necessarily mean it's a worse value. There are other factors like resale value, how long they last and how well they'll work for you and what you're used to. The Mac ecosystem alone is a huge selling point if you already use Apple devices like an iPhone.
I've been doing video editing and heavy photo editing on a Mac for years. I've owned everything from a low end MacBook to the high end pro machines like the current iMac Pro and old Mac Pro tower. I currently have a high end MacBook Pro and a base 27" retina iMac. At work I use a more loaded 27" retina iMac. One of the most important things to consider is plenty of RAM. If your processor and video card aren't at the high end of things, you'll still be okay for most things, but if you don't have enough RAM things can really grind to a halt. The better processors affect the speed at which it encodes video and how hard it has to work to do it. If you're not doing that day in and day out, it's probably not a huge deal if it takes a little longer when you finally go to export that video you've been piecing together. The actual process of editing it, splicing things together and what not, does not require a super computer.
As for photo editing, software like Lightroom is much more pleasant with a powerful processor and a good video card. The biggest benefit I see is the speed of loading previews when you're just flipping through things. The actual editing of the photo, not so much. Unless you're low on RAM, then it's a different story.
Back to the price/value equation for a minute. Take the current iMac 27" retina base model. It retails for $1699. I just bought one for $1563 with no sales tax via the University of Utah bookstore. Comparably, just to get a 27" 5k monitor of similar quality, you'll spend $1500. The next best option is still $1200-$1300. Just for the monitor. Yes, you can get a 4k monitor for $300, but you get what you pay for. Now get that $1500 monitor and go buy a similarly equipped PC and it turns out the 27" iMac is a phenomenal value. There are exceptions to this in their lineup, but when you really compare apples to apples (no pun intended) and factor in everything, they are not the notoriously overpriced machines that non-Apple users like to label them. Even the $5,000 iMac Pro cannot be built for less with the same PC components. This has been well documented online.
I recently sold my iMac Pro in favor of using my MacBook Pro with an external display when at home. I just don't do a ton of video editing and photography work at home anymore so having a $5k machine on my desk didn't make sense anymore. I bought a $600 LG 4k display to use with my MBP and it sucked. The image quality was okay, I guess, but not nearly as good as the one in the base 27" 5k iMac. And it was just kind of janky and really unattractive (yeah, that matters to me). I could no longer do a lot of the things that an all-in-one Mac does. Little things like easily adjusting volume and brightness from my keyboard. It required a bunch of other crap plugged in to get some of that functionality back. I looked at buying that high end $1500 5k display and I was just like "why not just buy another iMac??". So I did. The only thing I need to do to it is buy some aftermarket RAM and drop it in and it does the job fine. Is it my first choice for a video editing machine? No. Will it do the job for most people? Yep. Can you sell it in 3 or 5 years for WAY more than a comparable PC? Yep. I just sold my old MacBook that was made in 2012. It was still working great.
