Gengar is much better, even though Dusclops is the best thing around to screw up Forretress' Rapid Spin. Dusclops used to be a must-have, but its popularity has dwindled. You rarely see them nowadays.
I like Dusclops.
At least it can use it´s Attack and STAB for something, W-o-W is an awesome move, learns a ton of attacks to block with Imprision, great defensive Pokémon, just good.
It´s my favorite Ghost Pokémon too, while Gengar takes the 3rd position.
Dusclops is my favorite ghost pokemon. I like it's design and, like nightcat mentioned, it's thing with the black hole inside(Miroku style= damn cool). It's also great in battle.
I really like both. I use Dusclops on most of my team now. For Focus punch, Wow and for blocking Rapid spin. Hes fickkin awesome. Hardly anything gets past it. However, Gengar is ridiculously unpredictable. You expect it to use a certain moveset, you switch, and you find out its got another moveset. You then expect, it to be the moveset you thing it was using, but then you find out it has Focus Punch or something. I can never, ever guess what they opponent is using when I see a Gengar (which, mind you, is very often at the moment, despite Ice beam Blissey)...
Gengar is powerful and unpredictable, but it's not very durable, and has about as much creativity as a blade of grass does.
Dusclops, though, is creative in many different ways, and just looks a lot more thought-out and original than the pathetic Gengar. Gengar is obviously stronger than Dusclops, but Dusclops is still capable of dealing damage with a STAB'd Shadow Ball. It suffers in the same way Shuckle does(excellent Defenses, terrible HP), but not as much. And, unlike Shuckle, it can actually be a good Tank, with its two immunities and ability to block Rapid Spin.
Dusclops>Gengar by far. Dusclops can beat Gengar by dwindeling his HP with Wil-o-wisp while Shadow Punching away. Gengar can't do much to get passed Clop's awesome defenses.
I like gengar. Because I'm oldschool, and with a few exceptions, I'm just closer to the older, Kanto pokémon.
Whoever said haunter is better was right, though.
Gengar, because its appearance is really cool. It may not be the most original, but it sure looks evil.
Dusclops, however, is just ugly. Its one eye creeps me out, which is the last thing I want a Pokemon to do.