update and add a new line

You have an old plan. Adding a line to that plan may not be possible. Even if it is possible, two smartphones on that plan will almost definitely cost more than switching to a current plan. I presume the new line would be for your husband and you would keep the same number. You can definitely give him your 5s and get a new phone for yourself. Any new phone you buy from ATT will be full retail price. If you buy it on a Next plan, you pay it off over 24 or 30 payments so that would be an additional charge on your bill. Otherwise, the big question is how much data do you and your husband need. That will drive which plan works best for you.

Thank you so much!  I have no music, games etc on my phone nor would he.  Use phone for texting, email, FB, and phone calls, pictures and internet.  Hope this makes sense.  Husband had flip phone furnished him by where he worked but he just retired and now needs a phone for mainly emergency since he hates having a phone of any kind!  Thats why I was thinking about my 5s for him and upgrade for me but I don't need the newest or best.  I love my 5s and seems like 6s maybe good choice for me.  Would have to go to At&t store (which I hate) and they would have to set up everything for me....transfer my stuff from 5s to a new phone for me and set up my 5s with husbands info.  Does this sound like a  logical decision to you?  Thanks so much for responding!  

Sorry I did not answer about how much data would need....now have unlimited texting which I do a lot of...more than actually talking on phone.  And have several hundred pictures.  My 5s has 32gb whatever that means.

None of that has anything to do with how much data you need but based on your description of usage, a shared data plan sounds like it might work best for you. Shared data plans are unlimited minutes and texting and data shared between the two phones. Two lines sharing 3GB would be $80/mo plus taxes. Two lines sharing 6GB would be $100/mo plus taxes. You might also consider switching to a prepaid plan. You can get 6GB of data for each phone plus unlimited talk and text for 2 lines for $75/month on a prepaid multi-line plan.
Shared data plans are here: https://www.att.com/shop/wireless/data-plans.html
Prepaid plans are here: https://www.att.com/prepaid/plans.html

You might also consider getting either a prepaid iPhone 6 or iPhone SE from Best Buy. A 6 is currently $199, an SE is $149 and they will work on any ATT plan.