Single quotes are generally better.  With single quotes you only have to worry about "escaping" single quotes in the string.  I put "escaping" in quotes, because what you actually do is something more like this:
```sh
# Strings placed next to eachother ('foo''bar') without spaces are concatenated into a single argument
echo 'It'\''s Thursday!'
```

Double quotes are much tougher to deal with, because you have to deal with more [characters](https://www.gnu.org/software/bash/manual/html_node/Double-Quotes.html): `$`, `` ` ``, `\`, and `!`.

Note that these quotes are being done by `Symfony\Component\Process\ProcessBuilder` - they should quite definitely be correct.  The builtin [escapeshellarg](http://php.net/escapeshellarg) function does things the same way.