I prefer the Bitfenix Outlaw over the Merc for its wonderful design: the straight, defined edges of the dominant case work excellently, and enhance the overall look. In my eyes, the flat, even surface of the 5.25" drive bay covers are a superb touch, paired well with the case's deluxe design. Although I do admire the design of the Merc, it is a lovely case, the Outlaw excels aesthetically and feature-wise. Saying this, the Merc is that little bit bigger, which may benefit when trying to install a large CPU cooler.
The grill/mesh on the front of the Outlaw, reminds me somewhat of an engine. This gives the case a mean look, making it perfect to house my components - a case like this says, "a beast of a computer lays inside."
I also admire the originality of the inverted motherboard tray. It is an excellent idea; one that I have not seen elsewhere. I feel this will benefit greatly temperature wise and will allow for even greater overclock's, especially with the 8 fan mounts.