I would take what that PDF shows with a grain of salt. On the last page it puts the areca 1680 at 420 MB/sec read/writes. That seems odd considering my ARC-1280ML can do 830 MB/sec reads and 650 MB/sec writes and it is an older generation card.

Anyway, if you are going to use AS-series seagate drives neither of these controllers will work. I wanted to upgrade to the ARC-1680ix-24 but did not due to the fact that it has compatability issues with the AS series drives and I already has 12 of them from a previous raid setup.

One important thing (which I missed reading your post) is how many drives are you planning on having? I doubt any of the raid controllers will let you setup which drives are the mirror drives. Do you really need an external enclosure? This shouldn't be needed unless you are doing 24+ drives in your array.

Adaptec controllers I believe have a 16 drive (or was it 16TB?) limit when doing parity raid which disqualifies them for me.

I ended up using the ARC-1280ML since it supports the AS drives and I have had no issues with mine.