Last name?

It sounds like you're quite the lovely young person and that either family would do well to call you their own. What you'll want to do is that which makes you most comfortable. People tend to start thinking of themselves as one thing (i.e. name) or another and it's not so easy to change quickly. Tell your grandparents and your dad that you're thinking about it and that you cannot make any promises about such an important choice. Tell them you'd like them to give you some time and that you'd be happy to make up your mind by the time your 25. It seems that 25 or so is when most people tend to think of themselves as being adults, and so your wanting to wait to make up your mind to that august age will seem a sage choice.