I usually only travel with one other person so I never really look at luggage capacity....although the trunks look big and those drivers should know how to load the luggage effectively. I usually use Mears and I have been very pleased with them. Always on time, well dressed, and professional. Last time I decided to switch it up and use Happy Limo. While everything went ok, in my opinion they were less professional than Mears. For the trip back to the airport they decided to "upgrade" me to a Ford Excursion SUV limo. It was not very nice, seemed a bit dated, and was quite uncomfortable.
They also did not announce their presence when they picked me up to go back to the airport and since they "upgraded" me without telling me I sat there waiting for the limo I reserved while the driver just say waiting for me.
You will of course pay a bit more for Mears but IMO is worth it to avoid the amatuer opperations. Mears will give you a small discount for making a roundtrip reservation and non-alcoholic drinks with ice are provided. Mears limos hold 13 passengers.