I added a superclass Entity, which I inherited with my Ball, Enemy and Player classes. Now it's possible to compare classes.
I don't pass in Sprite as UserData anymore. I pass in the superclass Entity. I access the sprites through the class instances in my Draw() method.
In class Player : Entity
this.playerBody.SetUserData(this);
In class MyContactListener:
class MyContactListener : ContactListener
{
public override void Add(ContactPoint point)
{
Body bodyA = point.Shape1.GetBody();
Body bodyB = point.Shape2.GetBody();
Entity typeA = (Entity)bodyA.GetUserData();
Entity typeB = (Entity)bodyB.GetUserData();
// Ball collision with Enemy
if ((typeA is Enemy && typeB is Ball) || (typeB is Ball && typeA is Enemy))
{
// Do something based on the collision
}