The SPi's are a big upgrade in stiffness over the EX's, and thus probably better for steeps, trees and backcountry, unless you simply prefer a softer binding. You can get last year's (2009-10) SPi's for less than the price of this year's EX's, so that's what I would buy (in fact, that's what I did buy).
Your local shop is already discounting 2010-11 gear by 15%? Where is it?
Last year's SPi's, which you can find for less than $140, are still a much better deal than this year's at 15% off the list of $230, unless you think the changes for 2010-11 are worth the extra money, or you really want to support your local shop.