There is a definite difference in price for car rental in Houston, depending on which
airport you rent from. The price at IAH is more than the price for a Houston car rental at HOU, when searching rates for the dates that you are looking at specifically.
And, don't assume that all the companies have the same price range... there can be a big difference!
For example, I checked both airports for Aug 7 - Aug 14 for a midsize car (prices include taxes and surcharges, assuming same pickup and return time, no optional coverages and no discount code) and found a different rate for each company that I checked...
(opens in a new window) was the cheapest. At all companies, the HOU airport was cheaper than renting a car at IAH.
Make sure to compare bottom line totals too... not just daily or weekly base rates. You want to know the total, all in, including all possible taxes and surcharges for a particular company and a particular location!