Blue Bayou has a more intimate atmosphere ....mostly because it's dark (read romantic) and sitting on the water of POC. It is definitely fun to watch the POC boats go by (and have people on those boats waving at you!). There are of course a limited number of the "waterside" tables and if you're too far back you can't really see the boats, so if that matters, get to your reservation early and beg for a waterside table, lol.
Cafe Orleans is a brightly lit restaurant and there is both indoor and outdoor seating. I'm a people watcher so I actually like sitting outside and near the rail/walkway so that I can be entertained by those passing by. It can be a bit cool especially this time of year since it's outside, so be sure you have your jacket handy for dinner reservations.
Food wise....the Monte Cristo is exactly the same at CO as it is at BB, but several dollars cheaper at CO. Also it's not actually on the BB dinner menu, but you can request it. At CO you can get
Pomme Frites which I don't think you can at BB, and they're really really delicious. Addictive even, lol. The only other thing I've ever had was the salmon salad which is really delicious, but mostly salad with a rather small piece of salmon, so delicious but not super filling by itself.
Reservations are always a good idea so that you don't have to wait around or miss out if they're booked.....but don't write off just stopping by and asking if they have something available....we've done that at CO almost every trip and never been turned away.....not quite as good luck at BB, but I'm usually more interested in the food than the dark romantic atmosphere since I'm usually at Disney with my teens, lol.