Do you think it's better for a child to be a only or to have siblings?

Children need socialization with other children near to their age ...that doesn't mean they must be siblings, though.

Too many times if you are an only child, you are spoiled and don't share with others on many levels.
There is another problem later on. It's when you get married or live with someone. You really don't know how to deal with someone else being around except for having sex.
Doesn't make for a good relationship. Have them a couple years apart so that they can learn to play and share with others. Makes for a better person in many ways.

Among other things, it's good to have something to look up to.

Only child is much better. The parents only focus on one kid. They can not favour one kid over the other since there is only one kid. My parents use to favour my sibling, let them get away with anything.