Pricing a rental vs a shuttle --- We worked through the same thing (for our trip IN 13 DAYS!!), and found that up to about 4 days, the rental was virtually the same, once you figure in tips.
We got a great Dollar rate of $22.19/day on the smallest sized car.
We have used towncar/van services in the past, but with just the two of us, it will be nice to just grab the car and go.
Even with $5 or $8 coupons for shuttles, check the various rental agencies for discounted rental rates. There are a ton of good sites that keep up with discount codes as well.
But, we use Disney transportation while onsite. You just can't beat having the bus pull almost right up to the front entrance. And (at least as of now), you can't rent a monorail!