If I could only keep one, I'd keep the red -just because it's SOOOO rare and it's a very rich color that I would love to dress up my black outfits with.
That said, I actually own a dark silver and love it too. But they aren't as hard to find as the red, and they always seem to release new shades that are distinct each season, but still look a little the same, KWIM?
I would just keep the red, and try to track down a dark silver later, especially since they are releasing another shade this fall (not sure how different it will be from seasons past) -Good luck!