It is interesting if we think what the thought process for the left placement of the steam wand would have been when the first machine was designed. Was that person a left-handed (or) tried both the right and found the left convenient (or) as Jake said the lever of the E61 drove the decision? The E61 evolved much later, did it not ?
Personally, my decision was based on an open space on the right. I can stand on the side of the machine and steam. My earlier choice was Minima arm. But, @hysaf advised that Minima is good only on the left. So, I choose the Vesuvius arm. The Mantis arm would be too low a position, I think.