Emery Boards and Metal Files
Is one better than the other?
It all depends on what they are made of. Metal files have been given a bad name because the inexpensive kind are too harsh for your nails.
On the other hand, inexpensive emery boards are often glorified sandpaper.
How to Choose
Look for the emery boards made of foam. One side will be more coarsely grained than the other. One side is for shaping, while the other is for smoothing. They will cost more than a package of emery boards, but they are easier on the nails. Unfortunately, they don't last forever, so replace them often.
Look for files that are coated with diamond or sapphire dust. While the dust is extremely hard, it is also very fine and they should give you a lot of mileage for your dollars.