Hi everyone, I just completed 45 minutes of testing on a tripod in low light using objects of various colors and contrast. The 7D was in manual mode so all the settings were the same. I also tried to keep the zoom level the same on both lenses.
What I found was both lenses performed about the same in low light as far as the AF focus went but the 24-105 did indeed perform slightly better and was able to lock on a handful of shots when the 28-135 could not. I have no idea why the other night I had such issues getting it to lock. Granted I wasn't comparing the lenses head to head at the time. It must have just "felt" slower to me. Anyhow, thank you all for the input. I'm happy to say my lens is peforming well.