I had the OWC and currently have 2 of the Sonnet cards with 2TB Samsung Evo Plus blades.
From my testing the OWC's performance is erratic. With Blackmagic Speed Test the OWC 8TB, RAID 0 SoftRAID was getting 4.5GB/s W and 5GB/s R - but NOT consistently. The OWC was all over the place: 4.5GB/s W then the Read would drop to 500MB/s for a second then up to 1,200MB/s then settle at 2,300MB/s. Then the Write would be 3,100MB/s, Read would be 500MB/s to 2,300MB/s again. Then the Write is back to 4,000MB/s and the Read is 5,000MB/s.
The Sonnet card consistently showed 3GB/s+ for a single Blade and 7,400MB/s W and 8,510MB/s R for an 8TB RAID 0 array using SoftRAID. The Sonnet and the Samsung blades NEVER showed the extreme drop that the OWC did - the Sonnet barely dropped at all.
I haven't pushed the cards just yet during normal use but I have been copying large amounts of data to them and while the Fan does come on at times, it's not very loud. The Mac is so silent that I hear the hum of my old Samsung Monitor over the Mac Pro AND the Sonnet. And that's just a PSU hum.
I returned the OWC, it's very inconsistent. You might not 'notice' the drop because even 500MB/s is still SSD speeds, but it's just not acceptable for a blade. It's possible this was isolated and I received a bad card but in any case the Sonnet speeds are definitely better. I don't plan on using these in a RAID config, so in theory I'll be getting the 12GB/s concurrent speeds when all the blades are running.
The price difference is just going to be a personal choice here but I was not impressed with the OWC card so I couldn't justify spending 'anything' on it.