a sealed trait is the same as a sealed class ?

As far as sealed goes, yes. They share the normal differences between trait and class, of course.

Or, if not, what are the differences ?

Moot.

When is it a good idea to use a sealed trait (and when not) ?

If you have a sealed class X, then you have to check for X as well as any subclasses. The same is not true of sealed abstract class X or sealed trait X. So you could do sealed abstract class X, but that’s way more verbose than just trait and for little advantage.

The main advantage of using an abstract class over a trait is that it can receive parameters. That advantage is particularly relevant when using type classes. Let’s say you want to build a sorted tree, for instance. You can write this:

sealed abstract class Tree[T : Ordering]

but you cannot do this:

sealed trait Tree[T : Ordering]

since context bounds (and view bounds) are implemented with implicit parameters. Given that traits can’t receive parameters, you can’t do that.

Personally, I prefer sealed trait and use it unless some particular reason makes me use a sealed abstract class. And I’m not talking about subtle reasons, but in-your-face reasons you cannot ignore, such as using type classes.