Hi Dan! I work for Dell's Data Protection group and although my main responsibility is AppAssure, I can talk to you about NetVault. A couple of big differences between the two products
NetVault Backup also supports the EMC Data Domain backup and recovery platform, as well as its DD Boost technology for deduplication. Extensive storage options — Back up to a wide range of storage targets, including disk arrays, tape libraries and third-party deduplication appliances. Get advanced data protection for information.
I like your traditional/synthetic explanation. :) NetVault is really geared at the big enterprise environments, and because of that it can do things AppAssure can't: more OS support (esp *nix), native support for just about any tape library you can think of (although we do use a defeatured version of NetVault to provide tape support to AppAssure and vRanger).
If you only have 5TB, NetVault may be overkill, depending on your other requirements. What are your RPOs and RTOs? Of all of our products, AppAssure has the best of those (up to 5 min granularity on RPOs and depending on how you set things up, the potential for minutes on the RTO side). What other requirements do you have? What does the environment/applications you need to protect look like?