I think a town car service is probably your best option. I just booked, family of 5, a van thru Tiffany Towncars to the Dolphin for $140. That also includes a complimentary 30 minute grocery stop which came in very handy our last trip. The reason we booked the van (SUV) is because it was the only choice that offered both a booster and car seat. If you don't need both you might get by with the Towncar for $125. Not sure about the limo, but it does cost more.
If I remember correctly Mears shuttles,per person, were $22 one way or $34 roundtrip. Once you reach 4 people a Towncar service becomes a better option. Not sure about a taxi, but for a family of 5 it won't be cheap.