A wig is a full complement and covers the entire head, while a topper is a partial complement and is designed to fill the gaps at the top of the head. The topper should have a color similar to your hair color, because it connects to your hair from the bottom, it doesn't matter with a wig, you can choose any hair color that you like.
In the case of a topper, it should be similar in length to the length of your hair, possibly a few cm longer, when it comes to a wig, you have full freedom of length - whether it's a short bob or beautiful long strands.