So it's a bit old, but I can weigh in on this thread in a bit more detail now, and ask about others' experiences.
I did have a P20 Pro - for about three months before I lost it. As my insurance didn't cover the full value, and I had a high voluntary excess on my house insurance, I got a Mate 10 Pro as a cheaper replacement.
Obviously the P20 Pro is the better phone, and I do miss it. But I don't miss it to the tune of £250 (or I didn't at first, but... see below)!
As others have said, the battery life on the Mate 10 Pro is great - just as good as the P20 Pro as it has the same 4000mAh battery.
The phones both feel the same in day-to-day use. I did prefer the front fingerprint reader on the P20 Pro, but I guess that's just a personal preference (and possible just what I've got used to).
Despite the 0.1" difference in screen size, the P20 Pro has noticeably more screen real estate, and the Mate is feeling a bit cramped to me at the moment. Being able to hide the software buttons on the P20 and having the display bar in the notch gives an amount extra vertical screen space that I can definitely notice in general use.
The trade-off here in favour of the Mate is that it's a bit shorter and a bit easier to hold (the P20 Pro is pretty long and quite top-heavy and I did have a few heart-in-mouth moments when I thought I was about to drop it).
So, these are pretty minor differences for such a big price difference. Until we get to the camera.
Oh, man... the camera!
Obviously the P20 Pro is renowned for its camera hardware and once I'd got used to taming the software I was taking consistently great photos. My first set of shots on the Mate have been... not great. To put it very mildly!
I was pleased to see Huawei had updated the Mate 10 Pro to include Night Shot - a similar option to the P20 Pro's Night Mode. Or so it seems. Night mode on the 20 is astonishingly good. A real jaw-dropping feature. Trying it on the Mate was... well, a mess. I ended up with the most bizarrely saturated and shifted colours. Just all pinky-reddish and way, way over-saturated. Nasty, ugly photos. It may be that the Mate's hardware just can't really hack this mode.
Oh well, I didn't think it would have this option anyway, so no loss, right?
Well, it wouldn't be - IF I could get decent pictures in general. This phone seems to want to over-brighten every shot I take in low light (instead of using the flash). This also crushes detail in lighter tones. A lot of shots have an unpleasant amount of noise in the image too. I have to force it to use the flash in mid-lowish light conditions, and this is only possible in some modes.
Switching to pro mode allows much more natural-looking photos, so the hardware seems okay, but I can't find a single setting outside of pro mode that gives me decent results. Maybe I've been spoiled by the P20 Pro's camera, but I thought the Mate 10 series also reviewed well for photography? Am I doing something wrong?
Despite the positive aspects of this phone, these issues have left me a bit disappointed - I don't want to have to manually select ISO and shutter speed settings for every photo I take!
Can't decide. Leaning Pixel but there seems to be lots of people needing to get 2/3/4/5 devices before getting one without issues. Can't be bothered with the hassle so I just end up sticking with my **** phone which annoys me every time I take a bad photo (i.e most photos)