You can follow below link to understand about both the breeds and then decide which one would be appropriate for you and your family.
Getting home a dog is a huge responsibility and you need to be absolutely sure if you can invest proper time in his training, health care, exercise etc. Read up thoroughly about how to take care of them. Both of these breeds make great family pets. However, since you are a first timer and want to select only from the mentioned breeds, I would suggest you to go for Labrador. Rottweiler's can have aggression issues if not trained properly so not advisable for beginners. Having said that, any dog's temperament is a reflection of the environment he's kept in and the way he's treated.