The 27 is better finished
Better, in this case, is an opinion. I happen to prefer the Satin Blue finish of the M28 over the High Luster Blue of the 27.
The high luster finish takes more polishing (more finish time), and so is more expensive. Many prefer it, I do not.
As mentioned, other differences include the grooved top strap on the 27 (to reduce glare) vs. the bead blasted flat finish topstrap of the 28. Again, more work, more time, more cost on the 27. Fuctionally, I have found no difference.
27s came with target hammers, target triggers and red ramp & white outline rear sights. 28s did not. Original stocks on the 28 are the small "magna" type, on the 27 they are the large "target" type.
28s were only offered in 4 and 6" barrels, 27s in a wide range, from 2.5 to 8 3/8", and while the 4 and 6 are the most common, the other lengths are out there, and generally today, bringing a premium price.
27s were available in nickel finish. 28s were not. (there is a documented exception there were 5(?) 28s factory nickled for a specific order.) If you find a nickel 28 odds are very high it was done aftermarket.
I had 4 Highway Patrolman before I retired a couple years ago. Have sold two of them, at well below going rate in my area, and still a couple hundred bucks more than I paid for them. I might sell another, someday. My main one, not for sale, ever.
As far as I'm concerned, a model 28 is the best .357 Magnum you can own, in terms of what you get, for what you pay.