Q:

What's the preferred way of writing units per unit?

Why do we write units such as meters per second as $\mathrm{m\,s} ^{-1}$ instead of $\mathrm{m/s} $. Is the latter still a valid way of expressing units? Why are there two different ways of expressing the same unit of measurement?

A:

It seems to come down to personal preference more than anything else.  The exponent notation could be argued to be slightly clearer in a purely linear setting like browser text, since it eliminates any potential ambiguity over whether you're using the forward slash as a division sign or a separator.($\mathrm{m\,s} ^{-1}$ is more useful as units get more complex as it removes any ambiguity around the 'division')
Both are still used because they are perfectly correct. According to the rule of indices (exponents), $\mathrm{m\,s} ^{-1}$ and $\mathrm{m/s} $ are the same mathematically.

