For me it’s age and hand size. The SE is great, both my daughters have one and love them, but I find them hard to read text on these days and too fiddly for my fingers (though my hands aren’t exactly huge).

I had a 6+ for a while and got on with it fine but it’s a bit big. My 6s is just right. It is a little big for reaching on screen controls sometimes, but I’d hate to lose the readability so it’s a trade off.