Which 4* mutant arena boost to sell?

Otarius555

FreeToPlay_21

Both

XxLoganTDCxX

15mins

15 min
If your grinding arena an refresh champs, you'll get more value off the 30 min one.
And if Im not mistaken, the 30 min ones are more rare to get then the 15 min ones.

Deadpool87

Start using them during the Item Use events. Each of them is worth 100 pts and add up quickly, especially if you're hoarding the crystals in your crystal inventory.