Welcome to Four Daily Wins, your one stop reference for how to get the most bang for your buck in the Magic: The Gathering Arena Free to Play Experience.
Should I spend Gold first, or Gems first?
The Bottom Line
First, make sure you keep a reserve of 3,400 Gems to purchase a Mastery Pass during each Limited season, because it's an incredible deal for what you get. This is less important if you don't draft, but still may be worthwhile.
Once you have that 3,400 gem reserve set aside, as a general rule you should spend gems before gold on any event.
But Why?
The currencies on Arena have inconsistent conversion rates. This is by design, as it obscures the true value of assets on Arena and makes it harder for you to play for free.
In the Arena Store, all items are valued as though 100 Gold is equivalent to 20 Gems. But when you're paying for events, 100 Gold is treated as being worth 15 Gems.
This means that Gold used for events is more valuable than Gold spent in the store, and also that Gems used for events are less valuable than Gems spent in the store.
Therefore, when you want to enter an event and you have a choice which excess currency to spend, use your Gems.
This also means that if you ever buy anything in the store, including Daily Deals, you should spend Gold first.