The DX is made out of stainless steel, while the RG is made of brass. Some people think the DX glides on the face more smoothly. Lacking a DX myself, I have no idea.
The DX is the better one. It's made of stainless steel, as others have mentioned, but it's also a bit lighter and is thinner at the bottom where the blade sticks out. Since the mass of the holder is not your friend when skimming around the nooks and crannies of your face the lighter weight is good. And the thinner edge means you can use a lower angle when going against the grain, which helps make those blades a bit more forgiving, especially when shaving against the grain.
How much these advantages matter to you is a matter of personal preference; IMO if you're planning on using it regularly then the DX is worth the extra coin.
I have 2 DX's and I used to have an RG. I would have to say the the difference is pretty much cosmetic.
The Feather AC is now my only shaving system. I have used DE's and traditional straights.
I liked the concept of straight razor shaving, but I just could not afford the time or cash needed to keep a super sharp edge on my straights.
Unfortunately, I seem to have a very tough beard and not so tough skin, and my razors would have to go out for honing every 60 shaves or so.
I trying a Feather RG, and I knew fairly soon that the AC's were the right answer for me.
I used it and got more proficient with it for maybe 9 or 10 months. It was giving me the best shaves of my life and I decided that I had to have the aesthetically pleasing DX with the ebony handle. I sold my old RG, and after about a year, all but 2 of my straights and my old Futur DE were gone.
After about another year, I found that my mouse pointer kept hovering around the teak handled one and I could not convince it to stay away.
So at least, I am completely satisfied with my DX's and my shaving AD's are cured.
Please enjoy whatever you end up with.
Mike