Hi there,
I have 2 cockatiels and have had them both since they were only 4 weeks old, they are now a little over 18 months old, named Agro (male) & Buzzy (female)!
Buzzy was like that, when I first got her! She would shy away & run away to hide, hiss & bite, but... I found, the more time I spent with her, the better she became. I had to spend a little bit more time training Buzzy than I did Agro! But they were both successfully tamed/trained within a month or 2, with alot of patience, and persistence!
Now....some 18 months later...Buzzy is the sweetest, cuddliest little birdie I have ever met, and I, personally, would not swap her, or her Agro for any other bird!
If you DO decide on the Cockatiel, pop it in the cage and just walk up and talk to it for the first week (don't be surprised to find it hisses &/or tries to peck or bite you) Then start taking it out of your cage, let him/her get used to the noises and surroundings.
As a first time bird owner I am really glad I bought and persisted with m cockatiels, they are fully trained,& a very friendly, loving addition to my household
I HONESTLY Believe cockatiels are the way to go...they seems very easy to tame...and once tamed, are Very affectionate!
As for the Sun Conure...Unfortunately, I can't comment as I don't know anything about that specific breed...Sorry!