The cage I have for a cockatiel, lovebird parakeet is much diffrent then the one I use for conures, ring necks and if they were legal in my state a Quaker.
The cage is an honest concern, I'd reccomend buying a larger one if you decide on a Quaker though I don't know the exact size or bar spacing so I can't really give you an honest suggestion.
Its all personal really, but if you want a Quaker and the opportunity presents itself, I say go for it. There isnt really any place to start so you'll need to research a little extra before bringing home a quaker (they aren't at all like cockatiels)
I would say a Quaker is prone to having more personality both positively and negatively. From what I can tell Quakers are quite mischievous more so then the conures and certainly more then the Tiels. I've never met one in person (Like I said illegal here) but I have dealt with cockatiels and conures so I can say I would almost always go with the larger of the two (A conure)
I like the attitude they have, though let's be honest cockatiels can have quite the attitude as well!
Quakers are as good with kids as the kids are with them, so long as you supervise the kids around the bird and don't purchase an aggressive bird (Id reccomend buying an older one so you can avoid the 'terrible twos') you'll be fine.
1) I would say the Quaker is more friendly but that's mostly judged individually by bird to bird not species.
2) I don't know exactly how thin I'd need a number to know for sure, I'd buy a bigger cage to be sure
3) I have a sun conure right now and she doesn't bother me at all-- so I don't know you'd have to hear one for yourself
4) Quaker needs more attention but every pet bird should be given all of the attention you can give.
5) Yes, but again it's mostly bird to bird.
Best of luck :)