I went from 4s to SE just this month. Just like you, it took me a lot of thinking since I buy my phones unlocked upfront, I keep my phones for years.
What finally made me choose the SE as opposed to 6s+ are numerous, here are a few.
1. 4s to SE similar in size, and portability. I've become very accustomed to small screens. I keep my phones in my pocket or small purses. I move around a lot, the grip is important, I just can't hold and baby a 5.5 in in my hand.
2. I still love the old boxy design. The SE feels great in my hands, and personally I think it's so beautiful I don't even use a case with mine.
3. 3D touch is still new, I can wait a few years.
4. Except for the case, all the inside specs are comparable to the 6s. And SE's longevity will be just like the 6s. A very happy 3-4 years.
5. For basic comparison using 16gb models: $399 for SE and $749 for 6s+. I can't justify the huge price difference.
6. Yes, I prefer screen real estate, but for $350 more? No thanks. I think I'll use the money towards my next Mac.
7. I use my phone primarily as a phone, so SE fits the bill.
I was too lazy to upgrade my old 4s until I realized despite all the updates, my phone is functioning very slowly and really, it's been so many years, I want to get a new Apple product again and not feel like a cave woman. And I've been very happy with my purchase.
PS: originally got the 16gb SE, a week later I exchanged it for the 64gb.