You can become both a Vampire or a Werewolf on the same character, but you cannot be both a the same time. For example, you can become a Vampire first, then cure Vampirism, then become a Werewolf. That also works the other way around. You can repeat this process as many times as you want. I do prefer to level up both Vampire and Werewolf on most of my characters. If you cure Vampire or Werewolf, the progress of your skills will be saved. Therefore, the next time you get one of the curses, you will still be max level and all skills will be already leveled up.