I think part of your decision will depend on how many are actually coming from WDW and the airport(?) Also, where are you all going after? Rountrips are usually cheaper.
We have 7 in our group from the airport to the port for $150 in a limo or $85 in a van through a car service, this is only one way plus the tip that was suggested at $20. I have seen a limo for $125 and may be switching. We are renting a van afterwards because we will be staying over an extra night and would like to be able to get around. I believe the van is about $95, but my bil is getting it using "points".
We used the buses last year, airport to WDW to port to airport and everything went very well. There was a bit of waiting while buses were filled and people checked in. One nice thing was that we really didn't handle our luggage that much. There were only 3 of us and it seems to me that it was cheaper than a car service for all 3 legs of the trip. It really wasn't bad. Everyone on the bus is excited and on the way to the port, there is an informative video if you haven't been before.
Renting can be cheap with cars. I just don't like the idea of checking in and out, dropping off the car and so on.
Personally, I like the car service. They pick you and your luggage up. There is no waiting for anyone else to arrive. I chose the limo this time because my sister and her family are making their first trip to Disney and I want to surprise them and my son. They were also able to get a hotel free for us so it is a thank you as well.
You really need to figure out which method is best for your groups using different circumstances to see which one is the least expensive overall. If you are not concerned to much about the price, go for the car service (happy limo, tiffany towncar and there are others).