I have the iPhone 6 plus since launch. A week ago I finally upgraded to the smaller iPhone 8.
Main reason was the physical size and weight of the 6 plus is really noticeable to me so I wanted to try something smaller. Some pants I don't notice the 6 plus too much, but the recent pants (for some reason the pockets feels smaller) I really feel to the of the phone when I sit down. And most of my pants are from Old Navy and Gap for reference... The 6 plus in gym shorts is really noticeable. And in parks, if I have to run to my kids, I really notice it in my shorts and always hold the pocket with the phone. Even going down playground slide with kids, the 6 plus is really noticeable and always in danger of slipping out of gym shorts (which it has a few times in the past). When I use use it one handed, its always a balancing act. But the screen size is awesome and the battery life is awesome. Its pretty much the phone I want to use when I'm sitting down..but not the phone I want to carry around.
I recently went on a short trip out of town with just the iPhone 8. In any of my pants and gym shorts, I don't really notice the iPhone 8 in the pockets. Pulling out the phone is super easy, and its easy to access the whole phone single handed. Even for web browsing its fine (but I do miss the larger screen). The battery life is adequate.
I do occasionally get minor urge to bite the bullet and exchange for the iPhone 8 plus for the larger screen and battery though. But I really don't want to deal with the physical size and weight (especially it even heavier than the iphone 6 plus). That said, I did keep my iphone 6 plus (as my kids "media" device) and I do have an ipad mini so I'm fine with the smaller screen of the 8. If the iphone was my only iOS device though, I'd probably switch to the plus version.
There's no right on wrong with either phone. To me, its mostly a preference thing.