Retina display is incredible. Definitely worth it.

Cellular, requires a separate SIM card and of course a contract/payg package.

I bought wifi only as most coffee shops and hotels have it, and I use the tethering package on giffgaff (£10 a month) for those times I need to browse the Internet when I can't get wifi. Bare in mind, if she has an iPhone, unless she jailbreaks her phone I don't think it'll let you tether.

Extra space (64g) is always good some of the apps are particularly hungry on storage, downloading the hd stuff from iplayer can hoof it. I only have 16gb but I'm at the point I have to shuffle apps about sometimes to get space, still workable though just requires a bit of a cleanup every so often.