I've owned both the 6s and the 6s plus. More than anything, I enjoy the pocketability of the SE. The battery life is also a bit better than my wife's 6s. I'm very active (mountain biking, running, gym, etc) and having the smaller and lighter form factor of the SE makes it easier to keep the phone on/with me whether it be on a armband, fixed to a bike mount, in a pocket, mounted to a camera tripod, etc (I use a quad lock case).
I really enjoy being able to use it with one hand on the move. I also like that while using the phone up to my ear I have a firm/confident grip on the device and I never really feel like I'm going to drop it. It's great not holding the phone in strange ways and doing crazy finger gymnastics while using the phone's touch screen to operate the device lol.