hello bro, advance congratulations and anniversary wishes to both of you. 
for instagram and youtube usage it does require bigger battery. if she wants to use it for long time some days and not to search for any charger. considering 15 plus if budget allows will be good idea. else 14 plus suitable for most users.
getting as latest as possible is always good choice bro. 15 plus comes with all latest features and she wont feel the phone is outdated in next couple of years.