Depends what you want. The MC21 is very easy to derestrict and should cost less than the MC28. The MC28 costs an arm and leg to derestrict. You'll need a HRC card and have it programmed to you PGM for the 28. Whereas the MC21 is a simple wire splice.
However with that being said the mc28 is the more desirable of the 2 because of the PGMIV swipe card ignition and that there seem to be far less going around lately.

I own an MC28 and i would advise if you want a great bike and arent to fussed go for the MC21.