I don't think it will matter much. I guess maybe if your checking bags, maybe United "might" check them through but I doubt it. You'll have plenty of time to change airports unless.....you have a big delay on the inbound. Being August you shouldn't have big weather problems unless there is a Hurricane or something like that which would be rare of course. I recently had an AF flight out of IAH and used United to position to IAH which was fine. Ended up being delayed and only had about 40 minutes but I didn't have to change airports. Traffic can be bad in Houston but on a weekend it should help.