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Ok, I'll play.
In my opinion, a flatter swing is more reliable and more conducive to an inside-out swingpath. A flatter swing is also a result of age or bulk wherein establishing a vertical left arm, (a staple of the upright swing), gets harder to do as one ages, gains weight, or builds muscle mass as so many pro's are doing these days.
A flatter swing uses the large muscles of the body and as long as one can turn their back to the target, it really doesn't matter where the arms are because they play even less of a role in creating power. With an upright swing, the vertical left arm produces extension and a greater swing arc because the larger muscles are used to a lesser extent and the overall arc of a swing must be larger.
I think a flatter swing is much more repeatable and much more natural. Few players, (Nicklaus and Miller come to mind), have actually benefitted from a very upright swing.
Most of the best ballstrikers have a flatter swing plane because of its natural inside-out path. And because they're using the big muscles of the body, there's no need to "reach for the sky" as they say which, in my opinion, can really screw up an otherwise sound swing.
A flatter swing is less athletic and will last for years while an upright swing requires more maintenance and suppleness.
-JP | 
Tigers Swing Faults
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