Why is the M27 worth more? The M27 was S&W's premiere and "deluxe" revolver . . . with a checkered top strap, extra time lavished on a beautiful finish, etc. S&W only did these things on their Model 27 revolvers. This makes 'em bring a premium.
The M28 was the "working man's" M27 . . . a no-frills gun, with a plain finish that was every bit as great a revolver as its "pretty boy" brother. Lots of law enforcement folks trusted their lives to a Model 28. Great guns, both.
Sadly, both of the pictured specimens have been used a lot and . . .
The M27-2 in the photo looks like an early 1970s specimen before S&W started putting the wider target trigger and hammer on them all later that decade.
Here's a nickel plated 3 1/2" M27-2 from the same time period of the one you pictured that I own, but in great shape and with the correct stocks, "Magnas" (that were serial numbered to the gun). They are a LOT different looking than the wrong time period "diamond" stocks from an older period that the blued one currently wears.
The checkered topstrap . . .
Many folks believe these 3 1/2" M27s to be the best looking S&W revolvers of all time.
Frankly, I'd love to find a good "shooter" grade M27-2 of the same time period, just to enjoy shooting . . . but I wouldn't pay $500 for that rougher one with the wrong stocks on it. Somebody might though. Correct stocks, in good condition, raise the price of a revolver quite a bit . . . or detract from it.
Hope this helps.
Tom