They are very similar. If a personal opinion can help, then I feel the differences are more in software rather than hardware. Blackmagic adds important new features three or four times a year, and AJA does that more like once a year. Examples of this are the Photoshop export and import plug-ins, which are free for Decklink but require commercial software on AJA (or not available at all for import), the ability to do live keying of graphics, DPX support, etc. AJA IO users (myself) included had to wait about 18 months for the thing to work with After Effects and other compositing applications. But that's the IO, I undertand it's beter now with the Kona 2 range.
On the other hand, AJA products usually have more I/O options in a single solution, rather than the dozen model approach of Blackmagic. With Decklink, you buy the right model, or you'll have to buy a new one when you need that I/O option. It's hard to believe they don't have Y/C in any of their low cost models, when people in that price range are likely to find that useful.
Neither AJA nor Decklink can work with DVD Studio Pro to provide video monitoring right now, but I have reasons to believe maybe the new PCI Express models (both AJA and Decklink) could lead to that, for reasons I could explain if you want
Post Edited (10-25-05 22:20)
Adolfo Rozenfeld
Buenos Aires - Argentina
www.adolforozenfeld.com