I own a Skyscape X. Prior to that I owned the Refuge X, a tent similar to the Duplex (although it isn't fair to say they are the same). I would agree with the other comments — one advantage of the Skyscape is that it is very stable in the wind.
Another difference is that the Skyscape X does not have the tent Apex in the middle of the tent. I personally like this. It means that I have more room above my head when I lie down. I get a little claustrophobic, so I really like this feature (I'm often sitting in my tent to avoid the bugs, so it is nice to just lie on my back and have a lot of room above my head). Of course, the XPacks (like the old Refuge) is big enough so that you can sit diagonally, meaning you may not be that close to the roof anyway. Besides, you may find the idea of having the highest point in the middle of the tent to be a huge bonus — just sit up without scooting. Looking at the video (with a guy who is as tall as me) I would say that if height above your height is a priority then the Skyscape is just a bit better (but both have a fair amount of room).
Another difference is the pole placement. With the Skyscape X, the pole is on the inside. I personally don't like this. It makes it harder to put up the tent, especially in buggy areas. You have to open the door and put the poles inside, then go back out and make adjustments. It also means that if I want to use my poles for a day hike, it is harder to support the tent with a stick (I can do it, but it means worrying about catching the bug netting with the stick). I can bring dedicated poles, but I would rather not. I think an outside pole placement (like the Duplex) is better.
But getting in and out seems just a little bit easier in the Skyscape. The pole is part of the side wall, so when you unzip, you really don't have to worry about maneuvering around the pole (like you do with the Duplex). That being said, the opening is bigger with the Duplex. But still, from what I can tell, the "effective opening", the area you can pass through without worrying about the pole, is just a bit bigger on the Skyscape (due to the asymmetrical design).
These are pretty minor differences. In general, I would say they are both great tents. So basically, I would say that it is a trade-off. For me, personally, I find the Skyscape plenty roomy (as a solo tent) and appreciate the aerodynamic nature (and five stake design, which makes it pretty simple to put together). If the poles went on the outside, it would be a perfect tent for me (maybe someday there will be that option).