When fighting a dragon, I can either aim for the whole dragon, or pick a specific leg to target. What are the benefits of attacking the legs? Does it matter which one I pick?
According to this guide,
You can attack any of the dragon's limbs and his maw. It does not really matter but, if you land multiple strong strikes against one of the elements, the dragon may pause his attacks, writhing in pain.
For more clarification on what damaging the limbs does, this states
Both for dragons and giants if you inflict enough damage on the legs they end up resting on one side (dragons) or kneeling down (giants). It's very clear when that happens. They don't move in that case and you can dish out a lot of damage without fear of being attacked with the proper positioning. After a little while they go back to their normal state, so it's possible that, since dragons love to fly away and come back, the status was resetted or interrupted by such sequences
As for which to target, it doesn't seem to matter, though having a few people attack one while others attack another seems to be a common way of splitting its focus so that at least one group can take out a leg.