If I were you, to upgrade to a higher-end Aulos, I’d chose the 509B (“Symphony” model). I like it better than the 709B, which has an oddly placed right-hand index finger hole. An added bonus to the 509B is that it is the plastic model that is closest in size and finger positioning to the Bernolin resin 440. The Bernolin is somewhat fatter than most altos, including the various plastic and wooden altos I have. The Aulos 509B is also somewhat fatter. You do NOT notice this when you are holding them and playing them, but it’s just the way they are and it is nice to have a good quality recorder and a plastic one that kind of match up.
Here is a snapshot of the Bernolin vs a Yamaha Ecodear.
http://www.rahsoft.net/images_for_forums/bernolin-vs-ecodear.jpg
You can see how the Bernolin is thicker throughout (except the head joint which is somewhat smaller). The fat foot joint actually HELPS with the finger positioning because it makes the 2 lowest double holes easier to reach.
One other thing. Why do you want a 415? I do not think that that is a good idea, unless you play with a lot of pro-level folks or perhaps a harpsichordist who uses 415. You won’t be able to play in most amateur recorder groups, etc. Also, 415 altos are significantly larger than the 440s, so switching from the Bernolin to a 440 model will feel different.