Best to reach out to a knowledgeable Veritas partner. There are unpublished documents that cannot be shared in a public forum.
The biggest difference is the size of customer and environment that each one caters for.
Backup Exec is best suited for small and medium organisations with mostly Windows and/or virtual (VMware / Hyper-V) workloads.
BE also supports Linux and Some Unix OS, but should only be considered for small amount of n*x servers.
BE is also limited in the number of supported databases and applications.
BE has 2-tier architecture with a single media server backing up all the servers in the environment.
(ESO can be added to manage multiple BE servers.)
NetBackup is designed for large enterprise, with scalable 3-tier architecture (master with multiple media servers) to backup any number of servers/clients in the environment.
NBU has the widest support for OS's, databases, applications and modern workloads.
For more info, please visit the relevant product pages: