Overview:
xlsx is a newer format.
Code Samples below:
SaveAs xls
wb.SaveAs(fullPath, Microsoft.Office.Interop.Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
SaveAs xlsx
wb.SaveAs(fullPath, Microsoft.Office.Interop.Excel.XlFileFormat.xlOpenXMLWorkbook, Missing.Value,
Missing.Value, false, false, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange,
Microsoft.Office.Interop.Excel.XlSaveConflictResolution.xlUserResolution, true,
Missing.Value, Missing.Value, Missing.Value);