How well does the team know the domain in which the project rests? If it's a well trod domain, going to a new language (Scala) would do well to keep interest high, while not getting bogged down in the challenges of solving a known problem.
If, however, the problem domain is relatively unknown, I would recommend Java8, so you and your team aren't fighting two unknowns at the same time.
If, however, the problem domain is relatively unknown, I would recommend Java8, so you and your team aren't fighting two unknowns at the same time.