Best way to use money is to save it for events or leveling specific N cards in a hurry (mostly done during events, really).

An R gacha ticket is probably the best for you. It yields the best return results.

I'd say it is worth sticking around yes, heck many players in the start even stuck around in the very first part because the rate was 2 exp for 1 stamina. You can get them later but at a higher cost per spin.