Well you'd definately want to upgrade the front since they do most of the work. The Cobra's are bigger (the BAER claws I referenced above are 12", but they are slotted and drilled and match the back brakes). They are a little more at $470.
Where are you finding the Cobra 13" fronts for $399? I'm sure I've seen these as well, I'd just like to compare the two. Ck my link above, both the front and back brakes are on that page.