Areca was running a live demo showing 700+ MB/sec Read performance in both AJA and Blackmagic tests using only 8 drives in RAID 5 at NAB. CalDigit only claims up to 500MB/sec. So even if their both using the same Intel IOP, I quess the Areca engineering team delivers a better designed RAID controller around the Intel IOP to produce the higher performance. The 200MB/sec difference definetly compensates for the price difference.
I also saw the ARC-1680LP at their booth. It supports 4 internal drives and can connect external storage boxes, up to 128 drives if using a SAS expander box. This is definetly the best solution if you want expandability.