Well, I'm also in MD and have been to both BPG and BG. Although I haven't been to BG in a few years.
The date has changed for BG. it was in October for a while, but now it's the second week of Sept.
As mentioned above, I think both events have different things to offer. Bowling Green seems to have a real fun atmosphere. Even when it would rain, it was always a fun time back at the hotels and around town. The Corvette Musuem and factory tour are always on my 'to see' list. With the new Stingray, I wouldn't mind going back to BG and seeing that again. BPG doesn't seem as 'happening' in town, but from what I understand last year the host hotel and neighboring town held a car gathering, so that's cool.
Something to keep in mind... it takes me almost 11 hours to drive to BG. BPG can be done in less than six. So, that has been a deciding factor for me with gas prices. Although going in a caravan with other GNs to BG is a lot of fun.
Tough choice.