Amemar,

Anytime a manufacturer gives you a value for their product to use in your analysis, use it, unless you have reason to doubt their claim.

You can use the stiffnesses from the application guide (or the expansion joint modeler in CAESAR) when you don't have a specific manufacturer at the time of the analysis, or you're merely proving a proof of concept.

Of course, ideally, you "should" return to your analysis with the correct expansion joint values once a vendor is selected, but that's up to the individual paying you for your time.