I agree that different projects prefer different tools, but these data stores most certainly do compete with each other for developer's use within Google. Per

I agree that different projects prefer different tools, but these data stores most certainly do compete with each other for developer's use within Google. Per the paper: "At least 300 applications within Google use Megastore (despite its relatively low performance) because its data model is simpler to manage than Bigtable’s, and because of its support for synchronous replication across datacenters." So, the current trade off is ACID vs. performance, but I think the interesting point is that ACID is winning more and more now that a distributed ACID database is available to their developers. Again, according to Google: "We believe it is better to have application programmers deal with performance problems due to overuse of transactions as bottlenecks arise, rather than always coding around the lack of transactions."