Both have pros and cons, so whichever you feel happiest with will be the best for you. Of course, the only way to find that out is to try them! Online builders are generally easier to make characters with, as much of the heavy lifting is done for you, but pen and paper are more reliable and easier to adjust on the fly. If you're unsure which is for you, give both a shot until you settle on one you like better, or simply do what I do: use both! Nothing is stopping you from taking both a physical and digital copy of your character to your sessions, and it'll give you the best of both worlds. Just remember to update both simultaneously, or you'll risk confusing yourself on which represents your character at any given time.