Most likely, credits are going to be what you are running low on most of the time. There is a time shortly after you unlock mods where you can start stockpiling credits, but for most of the game credits are in short supply.
Galactic War Nodes will be your most consistent source of daily credits. Next on the list is the Credit Heist event. Having 5 strong scoundrels as fast as possible is critical in generating healthy amounts of credits. The Credit Heist happens randomly every 6 to 10 days.