1. Reduced distance to the sun meaning the amount of solor collected increases.
2. Less interference from atmosphere, clouds and smog
Cons.
1. The distance reduction is minute. The highest you could go and still be considered on earth is the peak of everest at 8.8 km. The earth is 150,000,000 km from the sun. The trig fails me but this is close enough to zero as to make no difference.
2. Reduction in things covering the panel should yield a better increase in voltage.
I'd say being in the middle of a desert, at sea level, would be much better than the top of Everest.
Also, beingcloser to the equator will yield better results than elevation due to angle of the sun's ray.