Jendays and Suns (And gold caps) are generally considered to be subspecies of the same species, that is, a species in the process of diverging into new and different types, which will eventually lose the ability to interbreed as they become further and further apart genetically, by which time they will be separate species. That is, if they were left to their own devices in the wild under the pressure of natural selection. It's all biology mumbo jumbo. The best way to think of them is say, like wolves and dogs, very closely related species that can (and do) interbreed.
The Gold-Capped, Jenday and Sun Conures | Voren's Aviaries Inc is a good one if you want more on it.
Being closely related (and some would argue with jendays and suns, colour/regional variations of the same species) they share many traits. I find my jenday Alex is very vocal and very loud. But his screech isn't as piercing or as high pitched as the average suns. I'm not sure if thats a jenday trait or just him. But for comparison purposes, they are roughly equally as loud. Which is, very loud. I've found it has been possible to let Alex know when his vocalisations are appropriate (during the day I do let him vocalise. I find it unfair to suppress his natural call and instinct entirely. At night or in the morning, he knows it is not on.) I also found by encouraging his softer chirps and calls, and his bad attempts at talking, he would rather use those to try to get my attention then the full conure scream, which also reduces the occurrences of calls. That said, a a jenday or sun is going to scream sometimes regardless, and it's going to be loud. And some birds become very vocal (given it is usually the owner accidentally causing this by reinforcing the behaviour)... Based on my experience, if you live in (or plan to live in) an apartment or even flat type setting I would advise against both the jenday or sun (I have no experience with the gold cap), unless the apartment was soundproofed. Not only walls, but floors and ceiling to avoid annoying those up/downstairs. In a house of your own though, I never have had any issues with neighbours and the noise. The only time my neighbours at my old place heard my boy was when he was outside in the sun, and being the middle of the day they did not mind.
I have never had complaints on Alex's vocalisations, but he DOES have screaming fits sometimes when overly excited. I find the best way to deal with this is to half cover his cage so he can calm himself down in the half dark. That failing, I leave the room until he is calm and quiet. Usually these scream fits happen after I have been away for a day and he is super happy to see me, and I really can't blame him (And I do let it slide when it is for that reason, because I am happy to see him too!)
I also find Alex great for an adaptable sleeping habit. Some nights I am out and up late, and will sleep in for quite awhile the next day. I never hear a peep from Alex while I am sleeping in! I do, of course get up and uncover him and feed him and then go back for my sleep ins, but even when he is up and awake when he would usually be being loud, if I am sleeping he is quiet as a mouse. Quite unusual for such a loud species of bird!
In all honesty the calling of my mum's cockatiels used to bother me more. Alex is one or two loud screeches, with the occasional screaminf fit, wheras mum's cockatiels were a repetitive shrill call that could go on for up to 10 mins at a time! And this would happen all day, every day! They were very happy guys, and very vocal!
I think loud is perspective. But in almost everyone's perspective, suns and jendays are loud! You can control it to an extent and minimise it by providing other vocal outlets and not reinforcing the screaming, but you will never get rid of it, and it's unrealistic to try. It's part of who they are. That said, you do hear of people with typically loud birds who hit the jackpot and have the quietest one alive!