Come on - this is kind of a silly question. The larger price decrease and the earlier the more impact on the sale Sony will get.
Dropping the price by $50 will achieve very little. It will still be $199 vs $350. Huge difference. The only way to make an impact is to drop the price by $100.
Of course, that will translate to $1B of lost revenue, which in turn will translate to 30,000 employess fired to make up for the lost, but I guess it is not what you asked.
Prediction made on 11/1/2008:
Q4 2008: 27M xbox LTD, 20M PS3 LTD . 2009 sales: 11M xbox, 9M PS3