I live in the midewest US and get a chance to try out quite a few Martins at local guitar stores. The D35s I've played have amazed me with their deep, resonant bass. The sustained chords just seem to bloom with overtones and harmonics. The braces aren't scalloped, but are thinner than on the D28

The HD28 will have scalloped braces and a "boomier" response in the bass dept. It's generally considered a flatpicker/bluegrass guitar around here, and the D35 would be more of a "folkinger" guitar - better to accompany "singers" songs as opposed to the punch and drive needed for bluegrass.

Of course, a good player will make either guitar sound great, so as many have said, it's personal preference.