I'd say if you do not attempt to load your movies and music on your ipad, then 16GB is fine. I've managed fine with my iPad 2 up to now, the only reason i ran out of space is installing too many games at the same time, some of the bigger games are easily 1-2GB a shot.
Rather invest in a PC/Media Centre type of setup which can stream movies & music to your device rather than copy it.
As for Nexus 7 vs iPad Mini , for informal usage, i don't think it matters much. You'll find the same apps on both. The biggest difference is the GAMES. iPad got better games and get new games faster, there is really no contest with this (since i got both ipad and nexus). The catch is, you need to do the whole fake USA citizen thing to get games (it's not in the SA store), on Android there is no such restriction.
In terms of hardware, the original nexus 7 got no front facing camera, the ipad mini does. The Nexus 7 got GPS in all their models, the ipad mini i believe only the 3G models come with GPS. So keep that in mind .
Also, you can plug in an external HD / flashdisk into a Nexus (with the right cable which costs like R50 and rooting) , while i can't say the same about the iPad .
I have however discovered the USB port on my Netgear Wifi Router, turns out you can plug in an external HD in there and voila, content is shared on the network. So if you need more space look into something similar, i'm still looking for a portable wireless router with such a port so i can do it more mobile instead.