The Digi 002 has its own AD converters for the four mic preamps, as well as for the line inputs (inputs 5-8). You could feed the Sytek's four line outputs into these line inputs, giving you a total of 8 preamps (the four built in, and the four Sytek). On the other hand, the Mackie has its own AD converter, which it sends via ADAT optical, which carries 8 channels of audio over an optical cable. The Digi 002 has an ADAT optical input to accept the Mackie's ADAT output. In this case, you would have 12 channels of preamps (the four built in, and the eight on the Mackie).
What your decision comes down to is how many channels you need vs the quality of the channels you need. If you're doing something like recording live drums, you probably need more channels, and the Mackie would probably be a better choice, even though its preamps probably aren't as good as the Sytek's. On the other hand, if you're recording things that don't need more than four simultaneous channels, say acoustic guitars, or rap vocals, the Sytek might be the better choice, because the need for less channels means that you can spend more per channel. Keep in mind, though, that with either choice, you can always add the other one at a later date. The Digi 002 can do up to 18 simultaneous inputs, I believe (8 analog, 8 ADAT, and 2 SPDIF).
One more thing you might want to consider is that, if I'm not mistaken, ADAT optical can only carry signals that are up to 48khz, so if you want to do 96khz you might be out of luck with the Mackie. I could be wrong about that, though, so hopefully someone will confirm it one way or the other.
Hope that helps!
