Never flown a DA20, but the DA40 is a very nice aircraft - one I'd be looking at closely if I was in the market to buy. Only drawback is it isn't turbocharged but that's not an issue for your mission though it could be for some others. (Turbo is VERY nice to have for crossing the Sierra Nevada or visiting Colorado - maybe essential - but not needed or useful for getting from the Central Valley to the Bay Area assuming that's your mission).
Cessna is better than Piper for the simple reason that there are a lot more of them. For some odd reason the ratio is much more pro-Piper in Europe. Also I've heard horror stories for parts for Piper lately.
The Grumman is no doubt a nice plane to fly - if you fit. I'm 6' and the one time I was a passenger in one, I had to fly it anyway because otherwise my knees were always in the way!