the e61 is considerably larger than the Dash and as such is a lot less "pocketable" Windows mobile has a variety of software to meet your multi-media needs and is really customizable if your needs for the phone ever change.
The build quality for the dash is pretty top notch, and is one of the best phones I have ever used. I held a e62 and was not happy with the plastic used by nokia.
That being said, I love S60 over Windows, the e61 screen is better by far, the keyboard on the dash is a bit too small (If you can palm a basketball, the keyboard is too small for you), and the jogdial on the dash is one of the most annoying inventions ever put on a cell phone.
I would say it's pretty even. I am happy with my dash and can't see having to go through setup again for an e61 as it doesn't offer much over what I already have or spend the extra money on an e61i as the camera is the same or worse than the dash.