flatslide carb vs roundslide carb

That's deeeeep water. I know that Keihin says a flatslide carb has better throttle response and will increase low end power. Why, I couldn't tell you, but since both Mikuni and Keihin produce flatslide (or D-slide) carbs as their performance models, then obviously that's the way to go.
 
I think that the flatsides are better for tuning and overall performance. I just do not like the round slides.