Math.random() v/s Random class

The Math class in Java has a method, Math.random() which returns a pseudorandom number between 0 and 1.
There is also a class java.util.Random which has various methods like nextInt(), nextFloat(), nextDouble(), nextLong()etc.

My question is that if I want to get a random number in a range (say, 30-70), then which way should I go? The factors under consideration are speed and randomness.