You guys have said the word "dark" enough that it really doesn't mean anything anymore.
RTD was good at the darker side of human nature. Midnight is the best demonstration of that, and it's some of his best writing. Cramped, claustrophobic, and wonderfully dark.
Moffat's good at a story that's dark in tone. The monsters that are always there in the corner of your eye is a running theme. The unease of being threatened by something that can't be seen is dark on a fundamental nature.
Now, as per writing quality as a whole, I think Moffat is more consistently good. RTD had high ups and absolutely abysmal lows, and Moff's run has been good to very good to excellent, depending on the episode. RTD is a great idea man, but falls through when the ideas are put into motion.
Also, Moffat's characters seem more realistic, to me. Like Rory. Rory's a normal person in the TARDIS. Or Van Gogh. Only very rarely is the "tortured genius" ever done well. Hell, even the Dalek cyborg guy had a bit of conflict to him. (I know that wasn't Moffat writing that episode, but the point stands.)
I think RTD's biggest advantage in making episodes was the phenomenal doctors. Eccleston and Tennant were both incredible, and they could usually make even a crap script watchable. When they weren't around to save it, though, you got Love and Monsters.