Get the one you want.
Blue.
the yellow fin up against the soft powder body is awesome. Plus i think most of us stive for more color in our tanks. Blue is one of those colors that eludes most of us compared to browns. I know my tank is green, orange and purple dominant, so having a nice soft blue cruising around set off the softer tones.
I forgot to add: If you go the powder brown route, be sure you research the fish and are certain you're getting the species you want. There are 2 species that are often called 'powder brown" tangs and one is much easier than the other. To further complicate it; different sources mix the two up as well. I think L.A. has them right. I'm too rushed (lazy) right now to check.