Foot file, pumice, or one of those electric things?

I want to sort out the hard skin on my heels properly and the shop had about nine different things for it, so I gave up and came here instead.

There is the cheap pumice stone, the metal file thing that looks like a cheese grater, and then a whole shelf of little electric buffers that spin. They cannot all be right. Some reviews say the electric ones take off far too much, some say the file is the only thing that works on proper thick skin, and one person online insisted a pumice is all anyone has ever needed.

What do you all actually use, and more importantly, how often? I am not after perfect feet, I just want to stop the heels catching on the bedsheets.