I'd probably look at the 5xxx series then, and compare those.
Other things to take into account: with 8 cams on an xx08, adding another cam (even not on the NVR but you want to record), you're doomed. The "surplus" for an xx16 is often marginal in comparison to an xx08.
2nd hard drive does also mean you can store (eg 4 channels) on the 1st, and 4 on the 2nd. In case of a crash, you still have 50% of your footage.
Another important parameter: you're going for a POE version or not.
Easy enough answer?
CC