I've owned and sold both. I owned the Blackbird (both base plates) a couple of years ago so I'm only going on my faint memory…
I found the Blackbird way too aggressive for me and frankly painful to use, it felt as though I was been skinned alive, especially using the OC base plate. This was a couple of years ago so (I'd like to think) my technique has improved since then. However I still found there was too much blade feel and I didn't like the way it shaved.
The Rex took a lot of getting used to and in the past I would've sold it on quickly. At this point (last summer) I'd tried a lot of razors and usually I'd use them a few times and if I didn't get along with them I'd move onto the next one. I decided to stick with the Rex and see if I could improve/adapt my technique.
The first issue I faced was ‘drag'. Drag is very noticeable with this razor and if you read through the forums you will see a lot of people have struggled with this. It's hard to identify why there is any drag but it is there. The way I overcame this was a very light touch and changing the angle so you are not riding the safety bar with the top cap just touching the face. Basically lifting my elbow higher (hopefully that makes sense?).
I would use it on settings 3.0 for WTG/ATG and 2.5 ATG. Anything above this and I found it too aggressive with too much blade feel for my liking.
I would get the closest shaves of any razor I've used and the finish was great, most of the time. However I would get a couple of tender spots even when trying to use a light touch. I also found the razor a tad too heavy and the balance was just a little off for me.
The Rex is a great razor but I never felt fully confident using it, which is a shame as I think it's a modern masterpiece. If they make one in titanium or Aluminium I would be very interested to try one though.
Comparing an adjustable to a non-adjustable is not going to be straight forward but if I had to choose I'd go with the Rex. I hope some of this helps?
Cheers
Rob
PS: Or you could just get a Game Changer? I found it better than both of them.