The 5500 series is top notch. The 7x code is quite different, but you get more interfaces, more memory, faster processor, and plug in modules to add features. I have put in a good number of these and everyone is always happy.
The 5510 is the direct sucessor to the 515. The 5520 may be overkill. It just adds a bit more capacity and the gig interfaces are nice.
Brent
Systems Engineer / Consultant
CCNP, CCSP