If it is an very late flight and money is no concern, I'd book the IAH on-site Marriot and worry about the rental car the next day. I stopped traveling to IAH five years ago, but it was nice to get on their lousy airport people mover and let it slowly just take me over to the hotel. (Meaning, there are better hotels for the money and this one certainly isn't special, but its on the airport). Just try and get in the goofy first building that is supposed to look like a 1960's idea of a space station because it's quieter.