I did not see this mentioned before but I believe there is another difference between the 520 and 550 not listed. I just bought a 520 for the bedroom and it does not seem to have a tilt sensor. I programmed a 550 for someone and I am almost positive that one did.

One other observation. The plastic seems a little cheapier on the 520. I got a great deal on it so it is fine for me but if the prices were closer to the same I would say definitely go for the 550.

Edit: I just pulled up the account info for the 550 I programmed and there was no option to turn off the tilt sensor. Can anyone confirm or was I just imagining there was a tilt sensor on the 550?