should I be more concerned with having lots of data on a mobile client/local storage? Or should I be more concerned with cutting down on the number of documents and number of calls to the DB (i.e. using pagination or infinite loading)
Emm, there’s no real difference between those ways to optimise app.
Ideally, you should do all: query chaching, document optimization, smart rendering and etc.
Normally, you try to limit clients data to what it needs now. So your topic question is almost never a case.