The cards are mostly identical.
The 7000 have the Rage Theatre (aka Theatre 100) chipset, while the 9000 has the Threate 200 chipset. The 200 is somewhat better, but not visually. It just offloaded more of the hardware compression to the card. Remember that ATI AIW is a hybrid software/hardware setup based on Ligos. That's one of the reasons the card was so excellent.
Some cards in the 9000 series had interference issues, mostly the 9600. But it was random.
There's no right answer here. Both are probably fine for what you need. My preference is to use a 9000 first, but it's honestly a preference based on nothing. My thought is that maybe the graphic RAM might matter if I do anything more advanced. (But I never really do.)
Your bigger worry is getting it to setup correctly. And then to setup and use
VirtualDub and Avisynth.