That's a good point Vinny. Kind of makes a person think twice about keeping tangs in hobbiest sized aquariums at all? I always found that "it grows to the size of the tank" nonsense that people always say about reptiles, to be quite irresponsible and actually offensive...
I agree with whoever said that purples always seem to look a bit healthier in the stores. Of course purples generally cost 2-4 times as much as yellows so I suspect that the purples get a lot better care during collection, holding and importation.
IME I would agree that purples tend to be a little more boisterous than yellows. Not much more agressive, but a bit.
I've kept one of each together in the same tank, twice in the past. In both cases they didn't beat on each other constantly, but they would get testy at times, and they would always seem to be "on guard", or in a constant state of alert/stress. In both cases the purple would get more food, stay fatter, and eventually out live the yellow. In the end I lost two moderately healthy looking yellows to unclear reasons (failure to thrive) and one purple to low O2 in a power outage. (The other purple is now living in it's fourth captive home in Westboro).