My husband and I are both upgrading our 5c phones. I got the new 7+ (its shipping this week.... Yay!) but my husband doesn't want the latest and greatest, he is happy with an older model. I have looked at the phones and obviously there is a price difference, about $4/mo for the 64GB version. Is the 6s really worth the extra couple dollars per month?
Just depends on what his needs are. The 6s is better than the 6 as far as speed and a few other things. Also keep in mind that if your husband is one to stay with his same device for as long as possible then he may want the 6s over the 6 because it's likely to be able to receive updates for at least one year longer. Ultimately I'd say go with the 6s but it's a judgement call.
It really depends on what the user wants from the iPhone. My Mom could care less if she had an iPhone 6, 6s or 7. Since she has had some medical problems I choose the models that has "Hey Siri" available, so she can call 911 without touching the phone. Take a look at this comparison, and really look at the features the user wants and then go to sites like www.swappa.com to compare prices. Remember that every year, the older model's value decreases, so do some research to stay ahead of the game.