If your budget is not enough for iphone_6s then the choice is obvious. Maybe now that new iPhone is on the go, 6s will get cheaper.
Wait for a while if it’s not an emergency. On the other hand, iphone_SE is a class on its own, is a 2016 model, I like it a lot to be honest.
The only thing that bothers me is the front camera. However, constantly be informed about the prices around and grab an eventual 6s bargain.