Difference between Document-oriented-DB and Bigtable clones

Can someone give a head-to-head comparison between them?

We are looking for a suitable storage engine for ou开发者_开发问答r weblog history data. We looked at Bigtable's paper and understand it is suitable to us well.

However, I also understand that Document-oriented-DB such as MongoDB seems to provide a little more powerful schema power -- i.e, it can model our data as well.

I wonder how nowadays ppl choose a scalable NoSQL DB --- I read enough articles like "we looked at A, B and C, and we decided to use C". But I'd like to see some benchmark number. What I am saying is that if MongoDB and the like can provide same level of performance as Bigtable clones, why don't web companies choose it (preparing to deal with various potentially more complex data problem)?

Thanks,

By the way, I read an article (which convinced me at the moment) saying Cassandra does not fit the M/R operation, any comments?

---

"I read an article (which convinced me at the moment) saying Cassandra does not fit the M/R operation, any comments?"

Cassandra 0.6 supports map/reduce. Your source was obsolete, apparently.

---

There's a not too detailed comparison here (notice the .pdf), but it's probably good enough to narrow down your search to 2-3 options.