First, I wouldn't call my child "a defective child". He or she would be, in my eyes and in how I described him/her, "a perfect child who happened to have a disability and/or a medical condition".
Whether or not I had (or adopted) another child would depend on how much love and care and time the first one required, in view of his/her medical problem and/or disability.
Since it isn't easy to watch a child have such a problem, and since it isn't easy for anyone to have to live with having "a problem" himself, before I had another pregnancy I'd want to know the chances of having another child with that problem. If the problem wasn't too extreme and there was high genetic risk of other babies' having it, I might consider having another child. If the problem was extremely difficult and/or painful to deal with I wouldn't. I'd probably adopt the next child if I could.
No child I ever had/had would ever get preference over any other; although a child with a disability and/or medical condition often takes up a lot of parents' time and attention, so I can see how other children might end up feeling as if the child with the problem was given "first preference". I wouldn't really be preference, but it could end up appearing to be (especially to the healthier child).
Then, too, sometimes parents know that their disabled/ill child requires a lot of time and attention,and that their healthier child often seems to take a back seat; so parents may try to make up for that by finding special time, or things to do, with the healthier child. So, it could look to someone else as if the parents sometimes "favor" that healthier child.
The only way I'd ever consider giving away my child with disability or medical condition would be if I were enough of a "defective parent" enough that my child's problem meant I've value him less out of my own ignorance and misguided values. If I found myself in that situation then - yes - I'd place the child for adoption by someone who wasn't "a defective parent" and could and would love my child the way any normal parent can bond with, and love, a child.