> Either system could support the other aggregation mechanism, and the only challenge would be in choosing which one to use.

Hive implements something similar to the paper mentioned. Partial aggregation on mappers & the reducer does a sorted final aggregation.

You'll find Hive beating MapReduce[1], even though it is implemented using MR.

[1] - https://www.cl.cam.ac.uk/research/srg/netos/musketeer/eurosy...