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Doesn't matter besides for cable management and type of air your PSU gets.
If you want to get technical, IMO bottom mounted is best when the port isn't covered. Reason being, the PSU never sees the warm air coming from the inside of your PC. Not only that, but most of the time cable management is easier due to being able to stretch that large 24pin cable and most of your connections are at the bottom.
 
also bottom mounted have the cables at the bottom; less leverage to trip; lower center of gravity for a tower to tip over.
 
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