I think this comes down to personal preference. I would rather not have to deal with the stress of traffic and insane heat. If you wake up an hour or two before your morning workout, the CNS effects shouldn’t be too bad. (Besides, the lack of stress from the traffic will translate to better recovery.)
I sympathize regarding the Texas heat, especially the last two weeks.
I think Quark’s right, you have to look at the whole picture. If you’re going to train in the morning I would suggest changing your schedule slightly to wake up a littler earlier to give more time for the body to warm up and for your CNS to ramp up. You might also extend the track warm-up a little.