- a bathroom scale (for weighing yourself)
- a tape measure (for measuring yourself)
- both – knowledge is power!
- neither – such data is irrelevant (or, if pedantic, such data are irrelevant).
I don’t actually own a bathroom scale. It’s a blunt weapon – it can only tell you one thing. A tape measure, on the other hand, provides much more specific and useful data, e.g. what size of garment to buy.