"...what can a 27 do that my 28 can't do?"
If you have to ask, you wouldn't understand.
A Cadillac and a Chevy are both cars, too...
I have a first year Highway Patrolman and the HP/M28 is a good, serviceable revolver. But it really shouldn't be mentioned in the same breath as the 27. The question really becomes "What can a 28 do that the 27 doesn't do with more style and panache?"