I have been to Disney several times and stayed both onsite and offsite. Have stayed onsite at all levels (value, moderate and deluxe). I have also stayed offsite many times. Here is my two cents...I recommend onsite for you and your sister. (Most offsite fans are larger families who want more space, bedrooms, kitchen, bathrooms, washer dryer, etc and are staying a week or more).
For off-site stays I would recommend renting a car. A rental car might be expensive because you are staying less than a week (weekly rates are more discounted) and you may be surcharged by the rental company because of your ages.
Offsite is also a good choice for visiting non-Disney attractions (Sea World, Universal,
Legoland, etc). Since your stay is short, I would stay on Disney property since you are only going to Disney.
The perks for staying onsite at Disney are great! Free transportation from airport to your Disney resort, extra-Magic Hours, package delivery, Disney theming, etc.
Now, which to choose...
As much as I really love the Deluxe resorts, they have become quite expensive. Unless money is no object and you plan to spend a lot of time enjoying the resort, a moderate or value might be a better fit.
If you are looking to save money and/or not planning to spend much time in your room the value resorts are a great bang for your buck! If you were to add a rental car (or transportation to and from airport) and parking at Disney to an offsite hotel, the cost/convenience of the Disney value resorts are seriously tough to beat! Rooms are small but for two people more than fine. (a family of 4 is a bit tight and 5 people or more forget it!)
Moderate resorts are my favorite! For a few dollars more, you get a bit larger room, more services and everything is a bit more upscale - sit down restaurants, slides at the pools, queen size (rather than double beds), etc. I do not think you can go wrong with any of the four of them. Just pick which theme you like - Caribbean Beach, Port Orleans Riverside (Southern theme), Port Orleans French Quarter (Mardi Gras) and Coronado Springs (Southwest). There are threads describing each one along with photos.
Good LUCK and enjoy your trip!
