Default S4/G2/Nex5/HTC1 decision?

New to the forums, my first post, & Happy New Year!

Long story short, I made an arrangement with a friend to add him & his wife onto my Family Plan, so that his wife could get a nice subsidized smartphone (poor credit kept them from obtaining post-paid service themselves). Since he refuses to use anything but a flip-phone, I also agreed to procure him a flip-phone whenever the need arises @ no cost to him. In exchange: he pays me $100/month for both of their lines combined, & allows me to use his Upgrade subsidy every 20/mos.

It's been almost 2 years, & his wife is due for her new phone. Since our deal hinges on her happiness keeping everything copacetic, I wanted to get some feedback before I pull the trigger. (As the "Landlord" of this agreement, I am also tasked with researching & obtaining new phones on their behalf.) That way he keeps her happy, & he only has to pony up the c-note every month. Zero research on his part, & that's how he prefers it.

Amazon currently has the: S4/G2/Nex5/HTC1 all available now for an upgrade price of $.01 (except the Nex5 @ $9, but I will happily eat that cost in exchange for his buddy upgrade in the future). Since they are all essentially "free", what are your thoughts?