The 3Ware Sidecar specifically targets Apple users; this becomes obvious when reading the datasheet and also in looking at the device, because its design goes well next to an Apple Power Mac G5. Since it is based on eSATA, though, the Sidecar can also be used with PCs, and the RAID controller cards by AMCC/3Ware are PC and Mac compatible anyway.
Like Accusys, AMCC/3Ware runs hardware accelerated RAID, which is based on the 3Ware StorSwitch architecture instead of an Intel XOR accelerator. AMCC uses only 128 MB instead of 256 MB cache, and it does not support RAID 6 with double redundancy. Differences cannot be found in everyday operation and management, but on the performance side: while Accusys provides better throughput, the 3Ware solution offers between 10% and 70% higher I/O performance, depending on the I/O benchmark profile and command queue depth. The Sidecar with the 9650SE hence is predestined for server environments, where I/O performance is typically more important than throughput.
The read performance of the Sidecar is mostly on par with that of the Accusys iRAIDer, but 3Ware loses ground on write performance, where it is approximately 15% slower. Another issue to consider might be performance on degraded RAID 5 arrays: 3Ware is considerably slower in this area.
There also is another advantage of AMCC over Accusys: the controller card is more compact, and hence can also be installed into low-profile rackmount servers. Unfortunately, the three-year warranty only applies to the controller card; the Sidecar only has a one-year warranty.