I remember that mc28 carbs have a smaller overall length which is good for top end performance and in conjunction with the shorter mc28 manifolds should give a better throttle response.
mc21 carbs are D-bored which I really like but performance over a standard round bore is questionable.
mc28 needle holder sucks but I believe that it can be swapped with the mc21 one.

That said, if you only have a HRC kit for the mc21 carbs, I can't see what's the point of comparing between them.
Obviously, the mc21 carbs are the way to go.
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