I thought my self the same question and I finally bought a second hand iPhone 6.
Most of my users are using iPhone and most of them >= iPhone 6, so it was the best price/usage/quality which I could afford. Still a price but sometimes it deserve an effort.
I’m not that agree with @JAR19 I’ve to say. I’m old fashioned, DevApp or any automatic testing are good, but a real test on a real phone is a must to secure quality.
But like I said, most of my users are iOS users. If most of you users aren’t, it’s another figure I guess.
And about your question difference between iPhone 5 or 6, the big difference is the screen size. When developing, I think the mixin of 6 is closer to 7 and 8 and the mixin of 5 is closer to the 4.
But this is only my point of view 