The ListDictionary
class is provided for the very reason you mention, and it's described here, and the suggestion is:
Recommended for collections that typically include fewer than 10 items.
Microsoft also provide a HybridDictionary
described here, to allow you to get the best of both worlds. It describes its typical usage as follows:
This class is recommended for cases where the number of elements in a dictionary is unknown. It takes advantage of the improved performance of a ListDictionary with small collections, and offers the flexibility of switching to a Hashtable which handles larger collections better than ListDictionary.
As for your specific case, the only way to see which performs best is to benchmark.
(Note that the examples above are for information purposes only! You will generally be much better off using the new .NET generic collections...)