Type and frequency of arena crystals the UC+ Grinders open?

I grind all the milestones below the five star feature and am trying to amass units. I open the uncollected crystal each time I have 1 or 2. Lately, however, this has been agonizing as I have gone 30 or so crystals without units. For those of you who grind arena constantly (and are uncollected or above, obviously) what type of crystal do you buy and how frequently?
Type and frequency of arena crystals the UC+ Grinders open? 22 votes
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One time I popped 15 at the same time (10 in one batch 5 in the other) and no units not even 15
There's also a fundamental misunderstanding of the arena crystals. They're the same as the difference between a $1 slot and a $5 slot, same odds, you're just betting more. With the larger bet comes a bigger jackpot though (5* Punisher).