There are issues of convenience as well as cost.
DCL transfers cost $35 per person, so if you have 4 or more, a limo service will be cheaper. However, you do need to factor in a tip to the porter at the terminal for your luggage handling too.
IF using DCL transfers you do not need to claim your luggage at MCO. You report to the Welcome Center with your luggage claim tickets from the airline, DCL takes the information and directs you to the bus. The poster is right that you might get on a bus and have it leave in 5 minutes or you might sit on the bus for 30 minutes while it fills. Each bus leaves as soon as it is filled, and they generally depart every 20-30 minutes. SO, you have the convenience of not needing to claim your luggage and the cost savings of no tipping for luggage.
If using a limo service, you DO need to go to baggage claim and wait for your luggage to come off the carousel. Again, that can take a few minutes or 15-20 minutes. You will then meet your driver near the luggage area and go to the car.
I agree that it is a 40-45 minute drive from MCO to the port, but that clock doesn't start running when you get off the plane. Does it really matter if you wait 20 minutes for your bags (not at all unusual at MCO) or wait 20 minutes for a bus to depart? I'm not advocating one over the other; just pointing out that the time from departing the plane to arrival at the terminal will be close to the same. In addition, when you arrive at the terminal from a limo service you will need to give your luggage to the porter who will confirm the info on the tags...again a few minutes of delay.
One other advantage; on our last cruise DCL was doing the entire check in process at the Welcome Center at MCO for people who were using DCL transfers (I WAS!) Seriously, they did everything--verified documents, issued KTTW card, took security photo, took credit card swipe, issued
Castaway Club lanyards and boarding passes. We had NOTHING to do at the terminal except go thru security and wait for boarding to start. No lines to stand in, no nothing. That is part of their preparation for the increased numbers of people on the Dream; they are also processing some check ins at WDW resorts for guests staying at a resort prior to cruising.
I really believe that your time till be about the same whether you do a bus or a limo service from the time you exit the plane till you are going thru security at the terminal. You basically have to decide where you want the "down time;" waiting for luggage or waiting for a bus to leave.