Do you have access to any discount coupons for the car rental companies? Costco, Sam's Club, AAA, Entertainment Book are a few options I can think of to locate coupons.
Then just go to the various websites and enter your travel dates, pick-up and return times, vehicle type and check any other appropriate areas that apply. Even if you have discount coupons/codes, try different options at each rental company to see what rates are available. Sometimes, there is a minimal rate increase from one vehicle class to another (economy to compact or compact to mid-size). It just depends on the vehicle reservation forecast for each vehicle class at each company.
If you are staying offsite, a rental car probably would be a good idea. Some resorts offer shuttles (most at additional cost), but their schedules usually leave a lot to be desired. Remember, WDW parking is $14 per day. Save your receipt if you park hop or leave and return to the same park in the same day.
Edited to add:
I *think* there is a taxi fare calculator somewhere--not sure if it's on these boards or floating about in cyberspace--that could give you a good idea of the cost of a taxi. Best guess it would be at least $20 per day, plus gratuities. I'm not sure where Emerald Isle is located or the taxi rates, but most of the rates I've been seeing in various places are $2.50 to $3.00 for flag drop, plus $1.50 or so per mile. You'll also have to allow extra $$ for traffic delays--the meter keeps ticking even when you're stuck in traffic!
