3up, I think it depends on where you think, after soul searching, your son really stacks up.
The WWBA events are much higher profile, especially for pitchers but also for others. If your son is a prospect for D-1 play, that would be my recommendation. Among other things it will help PG to get a bead on him as it builds his file. But the key is to open up multiple opportunities for your son, as opposed to just the few schools in VA who'll be interested in a player at his level, whatever level that may be.
I've never been to BIV, but my understanding is that it is a big D-3 scouting venue with some in-state D-1's as well. If you think that is where your son will best fit in the future, that may be a better value.
If you don't know for sure where he'll stack up, then go to the WWBA events this summer and see how it goes. The 17u event is crawling with scouts. The 16u is not as heavily scouted, but it will give you an idea of how he might do at the 17u in 2009.
My impression is that BIV is more focused on rising seniors, so waiting until next year shouldn't hurt him at all.