Can't say i've listened to john mayer...might have to now you've brought his name ot my attention.

Although jack johnson does make very nice music.
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Jack Johnson has much better songwriting skills, and John Mayer has much better soloing skills.

If i could only listen to one for an hour, it would be Jack Johnson.
Very different artists that I like a lot. But I think I'll go with Mayer - he's a much better guitarist and puts as much soul into his instrument as his voice. With Johnson I think his voice takes center stage too often and some of his guitar riffs are samey.

Also Johnsons new album sucks ass. I felt like I was listening to the same track all the way through. Nothing stood out at all.
John Mayer because he takes after the late, great Stevie Ray Vaughan and outs a great amount of soul into his playing, and also has amazing soloing skills as well.