I have the M8 and my coworker has the S5. If you turn brightness up all the way on both of them, there is no issue seeing the screens in direct sunlight. I live in Florida, and it is very sunny.
I will note that my M8 is slightly brighter than the S5 in direct sunlight. They are both great phones for viewing things outside.
I would recommend the M8 over the S5 anyday since the aluminum body is so much more premium in my personal opinion.
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