First thing is - why did you want a cockatiel in the first place?
What cage do you have and is it and all the things you already bought suitable for a sun conure?
To me it sounds like your breeder doesn't know what she is doing to be perfectly honest. The fact that he is so bitey to me means that she's made him fearful of hands and no one is respecting his space. Or, she simply isn't hand taming him like she says she is. A 6 week old cockatiel definitely can be tamed but if he's had a rough start to life, its an uphill battle at times.
Baby conures tend to love EVERYONE.
Now, here's some personal experiences. I brought home a cuddly little baby quaker, completely hand tame and well socialized. He was my bud. And then when he hit puberty, he started to bite me, fly away when I got near, and only wanted to be with my husband. It took me a good 10 months or so for him to even allow me near him again without biting. And its only been in the last 2 years of his almost 7 years that he has allowed me to pet him again.
Conversely, I adopted an adult Hahns Macaw that was hands off and I was told she would take AGES to warm up to me. Three days in she flew onto my face and then finally settled into the space on my shoulder, where she remained until the day she died basically. She was an extension of my heart and soul.
The point is, what you know now about both of these birds might change in a week or it might change in a year. Babies specifically are not set in stone - they are malleable to a larger degree. But all bets are off when hormones hit.
Mizzy, if you don't mind me asking, how is a hahns macaw? I've always wanted one, Seen one at parrot store, but they are hard to come by. Had many parrots in my life time, the hahns is one I found hard to come by.