Oooh, wonderful, thank you all for the feedback. I'm glad to have some people who can give direct comparisons between the species. Of course, it's tricky since there's so much individual variation, but it's still really useful.
I'd been leaning toward caiques again, but now I'm starting to think a GCC might really fit better with us. I have a feeling the constant energy of a caique might be too hard for my boyfriend to keep up with. I'd probably think it was fun and quickly get used to interacting with the bird that way instead of the relaxed playing I am more accustomed to. But, particularly considering that my boyfriend isn't as experienced with birds, I think he's more likely to be taken with one that will be willing to "take a chill pill" and be calm for a while and just hang out. I like having some of both - snuggly calm times, and some energetic interactive play. My tiel didn't do much of that. She was curious and investigative for a tiel, but really didn't play in a very energetic way like the caiques do.
Of course, I hear green cheeks are very playful, too, so hopefully I'd still get my fix on crazy-birdie-fun-times.
It's definitely still a bit of a toss-up, and I think meeting some more birds will really help us get a feel for them. But I think the caiques are a potentially riskier choice. I am pretty confident I'd be happy with the decision, but I think my boyfriend would feel safer with a smaller bird - less noise, less mess, less insanity.

We'll have to see!
Food motivation is a good thing. I -really- want to do a lot of training. Mostly, I'm interested in doing training that will make handling easier - get the bird used to being handled different ways, harness train, possibly flight recall, etc. But tricks and other behaviors are things I'm interested in too.
I wonder how the two compare in terms of being good with other people. I know caiques have a better tendency than most to be good with strangers or at least people besides the primary caretaker. But I'm not sure about green cheeks.
And harness training! I know it can be harder with smaller birds... that's something I am concerned about. I think harness training is one of my top priorities, because I want to feel comfortable taking my flighted bird out places, meeting people, etc. I feel pretty confident it wouldn't be bad with a caique, but even the smallest Aviators look SO big on the greencheeks; I worry that training would be pretty hard.