"Preference" is subjective, but the simple if
has a few advantages:
there doesn't seem to be a need to use pattern matching in this case, since you're not really exploiting its functionality (no extraction for example).
the second approach tells you explicitly that you check whether the list is empty, and hence is immediately readable.
In summary: pattern matching in Scala is a powerful tool, and because of its power it adds complexity to your code, both in terms of machine- and human-readability. In other words: not everything is a nail ;).