Java and Scala both compile down to JVM bytecode, so the difference isn't that big. The best comparison you can get is probably on the computer language benchmarks game, which essentially says that Java and Scala both have the same memory usage. Scala is only slightly slower than Java on some of the benchmarks listed, but that could simply be because the implementation of the programs are different.
Really though, they're both so close it's not worth worrying about. The productivity increase you get by using a more expressive language like Scala is worth so much more than minimal (if any) performance hit.
