Keep Brees. Manning is great but a lot has changed for the Colts. Also, Brees has consistently been a Top 3 QB (not that Manning hasn't). To be honest though, you'll be set at QB no matter which one you trade barring a ridiculous injury or meltdown by one of them.
I'm not to sold on D Will though. He had a great year but that Panthers offense isn't the greatest and they also have Jonathan Stewart creeping in to take some carries.