Newport is a little shorter and just a little more scenic than Point Judith, a little less city and more beaches, But either one is a great ride. Coming from Newport you will go up 14.2 miles of the east bay bike path on the way to Providence, much of it is on the water. evanyc's idea about 3 days is right. You need to add in ferry time and waiting and some stops along the way.
I have taken all those ferries with a bike, it's great. I took one from New London to Orient point, and rode LI from there ,it's great. Your trip is going to be awesome.

If you take the Newport route you will be going past three bike shops on the way. But they are closed on Sunday. But you will be going right by me too. Maybe I can give evan my cell phone number when you start up towards Providence, I have some tools and tubes etc, and a car. The end of March is when I really start putting on the miles up here. That's pretty good timing.

For a slide show of some photos I took of the East Bay Bike Path this fall go here and wait for the slide show to start.

http://www.dot.state.ri.us/bikeri/bikefall/bikefall.htm