The 8520 isn't a bad device either. That's why you have to choose from your needs/wants.
Here is a comparison chart:
Things I noticed off the bat were:
- Weight (Tour is the heaviest)
- Wifi (8900 and 8520)
- GPS (8900 and Tour)
- Avg. user rating (8900 has a higher one)
- Camera (8520 is the only one with less than 3MPs)