Wouldn't it be more effective to either call two different stored procedures (one for wide date ranges, one for narrow) or use the HINT option to force the JOIN type? That way you wouldn't incur the overhead of a RECOMPILE with every call of the stored procedure.

There's a good writeup on HINTs here: http://www.microsoft.com/technet/prodtechnol/sql/2005/frcqupln.mspx