IEnumerable IEnumerable is best suitable for working with in-memory collection. IEnumerable doesn’t move between items, it is forward only collection.
IQueryable IQueryable best suits for remote data source, like a database or web service. IQueryable is a very powerful feature that enables a variety of interesting deferred execution scenarios (like paging and composition based queries).
So when you have to simply iterate through the in-memory collection, use IEnumerable, if you need to do any manipulation with the collection like Dataset and other data sources, use IQueryable