Out here in Hill Country, live oak is prolific. Post oak comes mainly from the area east of here. Post oak grows much straighter and is about half as dense as live oak, so it's easier to cut and it seasons much faster. I helped a neighbor cut up a live oak that was downed in a storm and my arms and the Stihl got quite a workout. Some joints say that they use live oak and that it burns longer and hotter, which is an efficiency issue. I've used both and in the small relative quantities that a WSM consumes, it doesn't seem to make much of a difference. I do like the milder smoke flavor from oak, though. Texas barbecue is typically not as smoky-tasting as Carolina or Tennessee Q, largely because it's cooked with oak.
Jeff