I've been mid September and the two weeks before Labor Day. Mid September is definitely quieter, but I've found the crowds just before Labor Day were very manageable. I was at the parks this year on Labor Day, and it wasn't even particularly busy then. Expect long lines for the headliners, like you normally would, but most lines were under 20-30 minutes in 2008 and 2006. I think the longest we saw was KRR in AK, which had a posted wait of 40 minutes at about 12pm.
Although you have to bear in mind that a few people cancelled last year because of Tropical Storm Fay and the few others forecast to hit Florida. In fact, I think some days mid-September this year were busier than pre-Labor Day last year because of the storms. Also, some people report that crowds have been getting larger every August/September because of Free Dining, but I haven't seen a huge effect on the lines for attractions because of this.
I'm fairly sure that week is Value but someone more familiar with US Disney rates would be better off telling you, as UK travel agents charge differently.