If you only need the top 10, use rdd.top(10). It is faster, as it avoids sorting.
rdd.top creates one parallel pass through the data, collecting the top N in each partition in a heap, and then it merges the heaps. It is an O(rdd.count) operation. Sorting would be O(rdd.count log rdd.count), and incur a lot of data transfer — it does a shuffle, so all of the data would be transmitted over the network.
If you want to know more about Spark, then do check out this awesome video tutorial: