I recommend Sun Conure
When I was deciding on a bird, I was deciding between GCC, Quaker, and Dusky. The quaker concept went out the window quickly due to the fact that it was illegal in CA. Eventually I ended up getting a GCC because they were just more common, and at the time I didn't know any breeders who had them available. I do now. I just happened to find the GCC's ad on Craigslist and things just went from there.
However, I have since met Duskies. I don't have personal experience with them in my home, though. They are actually a rather attractive bird despite their dull colors. You wanted something with a more mellow temperment than a GCC, Duskies I hear are more easy going than GCC's. GCC's tend to be more argumentative as well as stubborn. They also tend to be slightly nippy. If I did it again, I may very well get a dusky.
I say this a lot, but I find it absolutely false when people try to market GCC's as apartment safe birds. Noise-wise, I couldn't imagine having my GCC in an apartment. I've had him for two years, and when he gets going, he does get going. It's nothing ear-splitting, and I actually can't hear him in my car when I come home (I can hear the Sun), but once I step out, he's LOUD. You can also hear him through the windows when everything is closed. I don't mind it, but If I were my neighbor, I would definitely complain.
I actually had a friend who had a caique and a GCC, and she had to rehome the GCC because the neighbors complained. I've heard of other people who have had very positive experiences with GCC's, however. So it really comes down to the bird you get. There's no way to tell for sure, either, before bringing him home.
The truth is that we could go on with blanket statements about these two species all day. But in the end, it all comes down to the personality of the individual bird you get, and as each individual is vastly different, blanket statements won't do us any good when it really counts.
Also, I am not a fan of cockatiels. They aren't as intelligent, and I find them rather dull. I grew up with one, and lost it a few years ago. That is my experience with them. However, if you are a more busy person, and don't expect to spend as much time with them, then a cockatiel is a better bet than a GCC or Dusky. I know you said this isn't the case, however. Cockatiel personalities are (again, blanket statement) less alterable than either of the two other species, and they are less likely to develop behavioral issues due to this.
If you remember your ecology, cockatiels are n-specific species, while more intelligent parrots are K-specific. This means that cockatiels mass produces babies to keep the species alive, whereas the K-specific parrots tend to rely on nurture and learning to survive. So, cockatiels are more akin to mice, and parrots are more akin to elephants. And so is their intelligence level.
I'm sure cockatiel owners would contest this, however.