As mentioned they are basically the same gun. The 28 introduced in the mid-50s as a lower cost working officers gun..... the 27 is the descendant of the Registered Magnum introduced in 1935 ( S&Ws flagship gun)
I have both a 4 inch +95% 28 and a 90% shooter grade 27....... which I prefer to having a collectors grade 27 to look at!!!!!
Be aware: the 28 was offered in both 4 and 6 inch barrel lengths throughout its production....... IIRC 6" guns are more common.
The 27 was offered in 3 1/2, 5,6, 6 1/2 and 8-something barrel lengths . In 1980/81 the 3.5 and 5 inch 27s were dropped from the catalog and replaced with a 4" offering.
So the reality is; the 4" 27 is one of the least common variations of the 27 as it was only offered for 10-12 years ; and it was competing with the new L-frame 586/686 guns.