This size thing really is funny the way it plays out. I posted earlier in this thread, saying how last year I started with a Plus, then moved to a 6. And this year I went right to the 6S Plus and that I've been very happy with it. But as much as I like it, after a week or two, the size is beginning to wear on me. Part of it is my insistence on using it the best I can one-handed. I often need to. Sometimes I'm holding something else in my hand. I might be my tea mug, it might be a flashlight at work, or some other tool. Sometimes it's a students iPad that I'm looking at while simultaneously looking up their login info on my phone.
So, I was content and determined to commit to the Plus for the year and see how it goes, but I'm starting to really miss the size of the regular 6. It really was ideal. Things I'd miss...
Screen resolution. I notice a difference, a significant difference. The regular 6S screen is still one of the best around, but the Plus is just better.
I'll miss OIS obviously. This is a big one. I'm very into taking video of my kids, and OIS is very, very nice to have, and it's becoming sort of a standard on the major flagships. I'm extremely disappointed Apple didn't try to get it in to the 6S, but maybe next year (which is another caveat that makes me want to keep the Plus, as I don't want to feel compelled to upgrade again next year in order to get OIS on the regular size phone).
I'll miss using Zoomed mode without it making the interface feel cramped and oversized. On both the regular and Plus, I prefer Zoomed screen mode. On the Plus it works out because I don't find landscape homescreen very useful, and I don't like the crazy-busy keyboard in landscape either. Zoomed mode takes care of all of that on the Plus. Zoomed on the regular iPhone, you lose a row of icons, and everything gets a little TOO big. It looks, clownish in a way on the regular 6 (you know, like a big red nose or oversized clown shoes).
I read a lot on the iPhone. Sometimes it's news, iMore, MacRumors. At work it's Google Sheets and Docs, and PDFs. I have terrific eyesight, but getting older and eye strain becomes more of a reality. My eyes definitely appreciate the Plus. More content on a screen, and larger, and crisper (there's that resolution/PPI thing again).
I don't use iMovie very much, but I have created little family videos right on my 6. It'd be way easier and more pleasurable working on the Plus screen.
The obvious battery life benefits would be missed, but not THAT much. My old regular 6 got me through my work day just fine. I took it off charger at 7:30. Worked a 7.5 hour day, with 30-50% battery left, and charged in the car on my way home from work, and got home back at 100%. I have a little Anker battery pack that I used to use when I was out just in case, but really never needed it. Maybe these benefits will seem more valuable during the warmer months when I'm taking day trips with the family and taking lots of pictures and what not.
As I'm writing this, I'm realizing that I really do want to stick it out with the Plus. There's things I miss about the smaller phone, but there's plenty I've written about above that I'd miss by leaving the Plus.
It's definitely not a clear cut choice, at least not for me. I can say though, that if Apple announced today that there was a 5.2" iPhone with OIS, I'd be exchanging my Plus for that. I really think it's just a bit too big. Not by a lot, but, if it was shaved down here and there, it'd be quite comfortable. Imagine the bezels on, and overall size of, the Samsung Note 5. Now shrink that screen to 5.5" (or 5.2") and keep the bezels proportionate. That would be an incredible iPhone right there.