I own a black capped (BC) but not a GCC. I've spent time with a friend's GCC, but he was not overly friendly. I can compare the BC to a parrotlet, since a family member owns one. And I can compare the BC to a cockatiel, since I've owned that in the past.
The biggest difference I've noticed is that my BC is quieter than any other birds I've had contact with. The parrotlet is grumpier and nippier than mine now, but when I first got my BC he'd nip constantly, hence my choice of forum name. Thank goodness we've bonded with each other and that has reduced the frequency of nips tremendously. Plus, my bird had a rough past and I think that shaped his personality.
The cockatiel was the noisiest in terms of constant chatter and whistles but it was mostly "happy" noise mixed in with some of those ear-piercing contact calls. My BC contact calls but it's quieter, less strident and not as often as other birds I think. I like how he greets me whenever I come back in my house. I have to go say hello and give him scratches.
BC re:nippiness. Everything would be fine, you'd be giving head scratches, then Suddenly "Squawk" and nip. Not happy with what I was doing, or hit a pinfeather wrong. Who knows? I will say my bird has very nice bite inhibition now, even the vet said as much on his last visit. So while he will get pissy and let you know, he's not drawn blood in a long time.
My two cents worth, bring home a bird that you feel drawn to and that shows interest in you. All breeds have pluses and minuses.