In general 6A is a better ride. Rt 28 sometimes seems to be a constant stretch of strip malls and parking lots with cars turning in and out all the time. Having said that, 6A is no picnic either - its narrow and twisty with lots of senior citizens driving along. If I was going to do it (and I have), I'd do it on a weekday and fairly early in the day.

I'd also look into using the Cape Cod Rail Trail as much as I could to get off the main roads. I'm not usually much of a MUP fan, but if it gets you where you're going, it might be better than battling with traffic. There are lots of organized rides that go from the bridges (both of them) to P'town - you might want to look at some of their route maps for ideas.