This isn't a battle per se but rather a test, which of these two has the better offensive/defensive weapons? Gaara's sand manipulation or Byakuya's Senbonzakura??
Hmm, they are very similar in how one wields them and their uses. Personally, I'd go for Senbon because they kinda do the same thing but you don't have to carry around that heavy ass gourd all day and end up with a bad back by age 26. Additionally, water has a negative impact on Gaara's sand but I've seen Byakuya fight in the rain with Senbon and it didn't affect him at all.
Million grains of sand or thousands of tiny blades eh? I'l go for sand simply due to it's sheer resources. Gaara has access to a vast amount of sand and it's mass & density should prove troublesome for Senbonzakura to penetrate, of course that is as long as we keep it at shikai......