I would recommend using the new GeoJSON format. Whilst I don't believe that any announcement has been made about dropping support for the old format, the fact that they refer to it as legacy should be an indication of their opinion.
There are some indexing benefits to using 2dsphere rather than 2d.
- Firstly it actually calculates queries based on the Earth being a sphere. One of the disadvantages of a 2d index is that it doesn't account for this meaning that you will have to handle the conversion yourself if you are interested in the actual area covered by a query rather than the basic lat/lngs.
- The ability to use compound indexes, if you want to do something like "get me 100 results from this area most recent first" then 2dsphere is your only choice.
- The ability to use geoIntersects queries.
- The geoWithin geometry queries require that you use the geoJSON format.
One other important thing to note is that you need to be sure the query you are using is supported by the index you use. If you use a 2dsphere for example you can't use a $box query as it won't be indexed - however mongo will not warn you - the result will just perform a table scan and will be very slow!
Mongo provide a compatibility chart of which queries can be used with which index