I've used flat and round and honestly once you're on the ice you don't notice any difference. The biggest factor I've found is where the horizontal bars lineup with your eyes... I've had some cages I couldn't use because the bar ended up right in front of my eyes, and others that are perfect. I'm currently using a couple old Jofa bubble cages but I also love the Bauer Profile II cages, they have a nice shape that's not too close to the face and fit my face well (the Profile III fits way different and sits much closer to your face, I hated it). I prefer a white or grey cage since I find I tend to notice black more when I'm not trying to (the oreo style ones are great, white inside and black outside).
Also remember that cage size and helmet size isn't the same, some people have longer faces and some shorter so if you can go to a store and sort of try them on that would be a good idea. I wear a large Bauer Profile II cage (which fits smaller than a CCM large but bigger than a CCM medium), and have worn them on small CCM V08's, small CCM 910's, and a medium Bauer IMS9.