Having worked with neither of them my perception is that MongoDB has "better performance". While I would never rely on that for making decisions about which to use, I'm sure that has some real consequences, particularly where search popularity is concerned.
Not saying it is bad thing to optimize for speed, it is just a bit dishonest to downplay the negative effects especially after calling a product a DATA-base.
http://nosql.mypopescu.com/post/392868405/mongodb-durability...
http://blog.boxedice.com/2010/02/28/notes-from-a-production-...