You get more colors out of an Ambanja but the blues on a nosy be are very pretty and come in earlier but they tend to grow a bit slower as you can see in my sig I have both lol
I prefer Ambanjas, personally. You can still get pretty blues but algo slip in some bars and spots in other colors. There are very high blue 'Banja lines out there, you just have to find the right Breeder.
There are blue ambanjas out there who are bluer than many nosy bes. I prefer ambanjas because I like the look of their rostral process (the nose). Theirs is more prominent than of the nosy be. Hopefully someone will correct me if I'm wrong, but I believe that Nosy bes have very subdued resting colors.