The speeds are 5 GB/s USB 3.0 and 6 GB/s SATA III (theoretical) but that mightn't be the only factor which would impact on performance. Maybe the controller in the ext. HDD would impact if it is poor quality (I know some cheaper enclosures have cheap controllers)? Or the drive speed? If you use those WD Green drives which alternate the RPM based on load it might affect performance. If you use a VelociRaptor @ 10,00 RPM it would be fine, but they aren't cheap drives. Even the WD blacks are pretty expensive (for 4TBs). Plus I would want to use an AC powered ext. HDD if I was connecting it to a console to store and run games from. I wouldn't want it powered from the USB port.
As far as I know, unless new info has come out, external drives for the PS4 won't allow game installs. You can put all of your multimedia content on it but no game installs.
I've seen comparisons between usb 3 and sata 3 regarding ssds and there was little or no performance difference. I could dig some up and post here, but i dont know if anyone is interested in that. Your right that things like the usb controller affect those numbers. Based on the reviews I've seen I didnt think performance would be an issue, but I was looking for more info on it.
I happen to have an external usb 3.0 case. It was an empty 3.5" case, so I can plug in any drive I want. I currently have a 4tb drive in it and so that spurred me on to investigate how I could use it with the ps4. The X1 info was easier to find since they made a specific statement on it.
Lets remember though that the hard drives built into both consoles are 5400rpm models, which means even a 7200rpm 2.5 or 3.5" drive could outperform it. Not only that, larger capacity drives have the potential to outperform smaller capacity drives. So if you had a 4TB 3.5" 7200rpm drive, it could outperform a 500GB 2.5" drive. Heck, even a 5400/5900 rpm 4TB drive could outperform 500GB 5400rpm drive.
When one of the merry band of MS apologists comes skipping into a topic telling us why MS made a decision without any evidence, it's kind of frustrating.
I agree, its silly for anyone come in and claim to be the expert on such knowledge. The truth is none of us know why Sony or MS make many of the choices they make. They aren't going to tell us everything. Its all speculation. I dont mind people giving their theory, thats the point of debates, but those that push speculation as the final word are going too far.