Amy,
That is a tough question. I personally love to not have to drive the car for a full week when I come on vacation to Walt Disney World. But I also know that the tradeoff usually is that I have less flexibility if I want to hit a grocery store for some room supplies, and transportation times may not be as fast as if I had a car.
For the day you described, if you used Disney transportation you would have some less that optimal routes. For instance getting from Pop Century to the Beach Club in the morning is not the most convenient, as you have to take one bus to Downtown Disney, then another to the resort. And if the reservation is really early, you might find bus service not as frequent as desirable.
Other legs of your day have great, scenic routes, and would be better than driving your own car.
So what is the bottom line?
Since you have the car, use it. You will get around faster, and have less headaches. You can ride the Epcot resorts-area boat some other day!
And yes, I wish I were coming along too!
Have a magical vacation!