The M27/28, M57 and M29 are all square butts. It is true the M28 is a more utilitarian piece than the 27 but comparing both side by side, there are other differences. I feel the 28 comes more refined from the factory, or at least it used to - smoother pull, outlined and red ramp sights, target trigger, target hammer and target grips. I had to 'dress' my M28 to get it near what the 27 felt like - but then I sent it back to S&W and they removed the old 6" bbl and installed a 3.5" M28 barrel and blued the piece to match - almost a 28.
At my local gunshop I see 27s and 28s in the case, each within $25 of each other but most right around $325. HTH. Mikey.