Aesthetics. Softail shocks are hidden under the engine/trans. and are made to resemble the old rigid (no suspension) frames.
Harley owners have been arguing the pro's and con's of Softails and swingarm shock bikes since the day they came out. I'm not gonna start another by stating my opinion (which is close to neutral). :thumbsup
Biggest pro for the dyna, is ease of adjustment. If you ride the same all the time soft tail is great, like if your always solo or always with passenger. That way you set the preload to where you want and forget it. If your solo half the time and with passenger half thebtime dyna is better sobyou can adjut the preload before each ride if needed. Other then that its mainly looks.