I would put my money on that BackupExec and Netbackup will one day be one product (most of the core comming from NBU)
The rest will just be licenskeys and features they open.
You can see it already. NBU Datacenter and NBU Business server is the same product. NBU can today read BE tapes (but not version 9.0)
Some of the schedule part in NBU 4.5 is taken from BE if I remember right.
Their is not much you can say about BE and NBU that tie them together except that Veritas owns both products and they do backup/restore stuff.
BE is basicly a low end (or mid sized) product from Seagate and NBU is a high end product from Veritas.
We only have 50 or so servers and we use NBU, but I know of sites that have more then 3.500 servers using NBU in one backup complex.
NBU has a lot of features, scales good, supports most hardware and no product supports so many different clients as NBU.
But the price is much higher and for a beginner setting up NBU is a hell.
But after you get it up and running good it's much more easy to handle and a lot more stable then BE.
By the way NBU performs a lot better then BE. A lot better.