Some points to consider:
- nr & type of interfaces needed
- throughput
- rack mountable unit required
- failover needed
If you need three interfaces or more, a 515E is the minimum. Gigabit will not be possible with a 506.
Check the specs against the required throuput.
The 506 is not rack mountable, all higher models are.
If you need a failover you should order an unrestricted license, unrestricted also gives you the growpath to the maximum nr of interfaces. With a restricted license, three is the maximum.
Regards,
Leo