NEVER EVER NEGOTIATE ON PAYMENTS.
If they can get your car payments on an STi within $50 of your WRX, you might take some time to review the loan terms. Don't forget to bring some astroglide so your ass won't hurt so much the following day.
If they can get your payments how you like them without you taking it in the rear, I'd say go for it. The STi is awesome. I still haven't driven one, and am keeping it that way to make sure I don't do something stupid and buy one when I shouldn't :|