I am mixed on this topic, before I adopted my first son, I would have said that you should not be able to choose because if you have a bio child you don't have a choice. But now that I have 2 boys, if we adopted a 3rd I would like it to be a girl. Although my husband always says that if another boy came are way, he knows I would jump right on it.
Now on the issue of one gender being easier than the other, I believe that it is all about the child's personality not the gender. I have 2 sons that are as different as night and day, my oldest presents me with a challenge every day, and my youngest throws the occasional fit but is very easy for the most part. And my oldest is a total momma's boy and my youngest loves his daddy, so you never can tell. But I will say has a mother of 2 boys I would never have it any other way.