Salamence has the cooler base design, but Metagross has a significantly better Mega Evolution...Salamence's looks like the Rebel Alliance symbol from Star Wars. Metagross has a much cooler shiny colouring, too - Salamence is green, which is the absolute worst colour shiny Pokemon can be. But between the two I've always used Salamence more because it's slightly easier to come by a Bagon than it is a Beldum much of the time, and Beldum itself is generally useless and a pain in the backside to level.
So base aesthetics and usability it's Salamence, Mega Evo and shiny colouring it's definitely Metagross. Salamence wins if only because Mega Evos are a one-off gimmick and I never get shinies. xD