We just went through this debate and we got 2. Here's what we have noticed, and keep in mind that ours are still very young.
They are sisters and were raised together by the same breeder and absolutely adore each other. When they are sleeping, they sleep snuggled together and it is the cutest thing ever! If you decide to go with two, we preferred to have them be related so that they are well bonded and comfortable together in the same cage. Up to this point, they have not had any issues being together, and in fact, if they can't see each other when they're playing, they will talk to each other. Nothing too loud, but from what we can tell it's just to confirm that the other one is still around.
For noise, they typically scream when they're hungry, but it's really just the younger one of ours. The older one is actually fairly quiet, but quite capable of being loud if she feels necessary.
In terms of them bonding to someone else, we have observed that they are mostly bonded to each other. We have introduced them to a couple of other people and they seem curious about people in general. Our research indicated that caiques in particular seem accepting to new people who don't behave aggressively or threaten them.
Overall, they have been such a joy to have!! We highly recommend getting them, particularly a pair if you can afford it. We got a pretty big cage and filled it with ladders and perches and toys so they can explore. They seem to really enjoy each other's company and love when we come home and get them out for play time. We would be happy to answer any other questions about them since we just got ours about 3 weeks ago and are still going through the process! Hope this helps