If you

1. Are interpolating a string - `"this is #{first_name}"`
2. Have a string with a single quote in it - `"He's right"`

Then use double quotes. 

Otherwise - use single quotes.  This isn't much of a philosophical thing but rather a consistency across the app thing. I believe I still have some places where I hadn't been following this so I make the changes whenever I see it in a file I open. 

One caveat - in HTML it's the norm to use double quotes so  stick to " " in erb / html. If you include a hunk of ruby code in the view, though, single vs double still applies.