Houston hobby is about 45 minutes away. There is bus service from the pier with a cost of $30.00 per person. The problem with the bus is that it gets you the airport somewhere around 10:30 -11:00am. If you are have a flight on southwest that is more than 4 hours later, you cannot check-in. Therefore you will be in the airport with nowhere to go and you will have your luggage with you and the pre-screening area is very small.
The reason for the 2 different rental car companies has to do with the open hours of the office. Since my cruise started on a Sunday, I arrived in Houston on Saturday to make certain I did not have any luggage problems, did a little sight-seeing and got a good nights rest and made the trip down on Sunday morning. The enterprise rental office is close on Sunday but they do have a return drop box. Unfortunately, the office is about 12 minutes from the pier.
For the return, since the enterprise office is closed on Sunday, I chose Hertz which has an office that operates in the cruise terminal from 9-12 on Sundays.
Hope that helps. By the way, if you are in the airport for longer than an hour, you must have lunch in Pappas resturant. Really great burgers!