The "break even" point on
DCL transfers is 3 people. If you are 3, cost will be about the same whether private or DCL...however, a private service will transport you on YOUR time schedule and will allow a 15 minute grocery or Wal-mart stop if you want it.
DCL transfers are convenient. However, that's the only good thing about them. If you are at WDW before the cruise, the bus doesn't leave WDW till after noon, which means you get to the terminal after 1. That's later than I like to arrive. We always plan to be at the terminal by noon (that beats the DCL busses). Doing that, I've never had any problem getting the tickets/ressies I wanted.
If you use a private service, they drive you up to the terminal where the porter unloads your luggage. The next time you see it is in your cabin. It doesn't matter whether you are coming from MCO or WDW.
If you go private from MCO to Port Caneveral, the driver will meet you in baggage claim and take your bags to the limo...then to the porter at the terminal...so luggage handling really isn't a hassle.
If you want, you can go private on the way to the terminal and DCL on the way back. That way, you can check in your luggage for the plane at the Port (on most airlines). You still claim your bags in the customs area, where a porter takes you thru customs then to your transportation (whether DCL or private.)