My wife and I have had our iPhone 4 phones since 2011, before we got married. We have been putting off upgrading over and over because the next one was always right around the corner. This year, as part of our Christmas together, we decided to finally upgrade. We put a lot of thought into it and every time we passed a display in a store we would check out both models. We even went to the AT&T Store just to look at the phone, to see what we thought.
We put them through the paces and held each of them in different ways. Heck, I even purposely wore my work slacks and my jeans on different visits so I could be sure it would fit in both. I couldn't put the actual phones in my pocket since they are tethered, but I did think to find the smallest cases possible, take them out of the box, and simulate the phone in my pocket by asking the employees if it was cool to slide the case in and out of my pocket. They agreed, and it helped a ton.
In the end we both decided to go with the 6 Plus model. When I talk on the phone I almost always use bluetooth, and I set my wife up with that as well as another part of her Christmas, so holding it to our heads wont happen too terribly often. We also use our phones for video, web browsing, and FB, and things like that a lot. For our purposes, it just made sense to go with the Plus.
I wont lie, I know that I have texted and checked out various things while driving and I'm not proud of it. That isn't really possible for me to do with this new phone so it has pretty much cut that out for me, which is really a good thing.
We have also been using Siri a great deal, especially while driving. Making calls, sending quick messages, and all sorts of different things. It's awesome to be able to do stuff like that totally hands free. This is our first experience with anything like Siri so we love it to death.
As for cases, my wife went with one of the pretty Sonix Inlay cases which barely adds much more size at all. I went with my old standard (my 4 remained in the case all the time other than to clean it and things like that all the years I had it), the OtterBox Defender. I wasn't originally going to get another Defender because I thought it would be WAY too big, but it ended up being just fine. This version of the OBD case is much thinner than the one for the 4, and it fits in all of my pockets just fine. I certainly don't wear skinny jeans, so that isn't a deal with me anyway.
This is super old news, but here are some comparison pictures I took of my 4 and my new 6 plus, both in and out of a Defender case...
(and yes, I know the case for the 4 has problems now, haha)