start by asking yourself the following questions:
- is your data format not well defined and likely to change - add fields that you don't know yet? Adding field in rdbms may lock the db or affect performance.
- is the data going to be big? 5-10 gb table in MySQL will not work efficiently.
- high insert load?
- nature of the data, eg. geo data that you want to query by locations, mongo has support
MORE ON: dzone post - when to use mongo rather than mysql