Either- depending on your current maturity and mind set. Casual - if you want to be carefree, single, non committed and only want the physical aspects of the relationship. Serious - if you want committment, responsibility, emotional as well as physical, a good friend and looking towards making a life with this person. I was quite happy with casual relationships until i felt inside me that it was time to settle, start a family etc. So casual was fine when i wanted to be single and carefree, but when i wanted committment and children, serious relationship is great also. My partner is my best friend.