I don't own both, but I have spent some time comparing the two myself.
The Courtenay has a more finished look, more sophisticated. The shape is gorgeous, and that outer turnlock pocket can really come in handy for things like a cell phone, keys, etc. It's also nice to have the two different straps. The crossbody strap is thinner, though, so it can dig into your shoulder a bit if your bag is loaded up. The bag is a little smaller, so it holds less.
The duffle, in my opinion, has a more casual look because it is less structured. The strap is wider, which is really comfortable. The outer zipper pocket can be a little harder to get to if you wear the
Coach stamp facing outward. As you know, you can wear this bag crossbody or double up the shoulder strap, but it will take you a minute or two to switch from one mode to the other. With the Courtenay, the switch is instant.
I know you love your duffles, soccergirly87! Throwing a Courtenay into your mix would give you a slightly dressier option than your duffles if you ever decided to change things up some days.
All this is just my opinion! Hope this helps. Let us know what you decide!