This is a very broad, general, open-ended question. Can you provide some more context as well as some details of what application you have in mind?
One of the ways to think about the use case differences would be along the lines of scale and query needs.
If you have simple key based queries involving single and/or range based predicate look ups and you think there may be a need to scale beyond 5-10 or so nodes, you will be much better off looking at Oracle NoSQL Database. Mongo is known to have scaling issue due to is 1 lock per database design which makes it a bad choice for a true scale-out solution. If you have a very value based query requirement where secondary indexing becomes critical to fast data retrieval, and you don't anticipate the need to scale too large, then Mongo is probably a good choice for you, though Oracle Standard edition may be as well :-) Keep in mind, we are coming with secondary indexes as a feature for Oracle NoSQL Database soon, so that choice will become much less obvious.