Which way is more efficient for generating supply in the long run?

1) Calldown extra supply on your depots

2) Calldown MULEs to mine more minerals and then use the extra minerals to build depots

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Q u o t e:
When they first announced calldown supplies it only gave +2 and they said it was for emergency use only. Currently, calldown supplies gives +8 supply. This means your saving 100 minerals(depot cost) plus the amount of min that the SCV can mine when it would have been building a depot.

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Calling down the MULE is much more effective in terms of collecting additional resources (each will gain more than +100 during it's duration) but if those supplies keep you at cap by mistake, the supply depot call down can quickly get you back on track for your build order/timed push.

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Q u o t e:
Thanks Karune :)
So will pros with set BO always just use MULE?

Do you know how much exactly does the MULE gather in its lifetime?

Does the Orbital Command have a cooldown like Gearvosh says? Do the queen and Obelisk?

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The amount that the MULE gathers is subject to balance, but it will be more than the cost of a supply depot by a generous amount.

The Orbital Command, Queen, and Obelisk do not have cooldowns. This means 'yes' you can drop down several MULEs all at the same time. The abilities for the Queen and Obelisk will not be as forgiving if you forget to use the abilities, as you can only queue one set of larva at a time per hatchery, and probes with additional probe buffs will not stack, but it will reset its duration.