David,
That's a groaner.... The trade is close...... Much depends upon the details such as; Is the 28-2 and "S" or an "N" prefixed frame? What kind of condition is the M&P in? Is it a "V" Serial Numbered gun? Is it Polished Blue, Brushed Blue or Parkerized? Is it correct (stocks, lanyard ring and all) and all origional?
Unless the M&P were in Very Good-to-Excellent condition my inclination would be to make the trade I think. The .38 S&W (.38-200) chambered guns don't seem to bring the kind of money the .38 Special guns do....
A 4" "S" Frame 28-2 with Diamonds is heading for $500 maybe a little more if indeed NEW IN BOX. An "N" serialed gun would be about $50 or so less.
Drew