depends on your flexibility I'd say. If you buy discounted tickets you usually have to buy them in advance and they are for one specific train on one specific occasion only. WIth the interrail you can come and go whenever you want. I still think the one I suggested (where you could travel on 5 days within 10 days) for 249 Euros /adult/ 2nd class, could be worth it if you use it from Interlaken/Bern on as going to Paris and back to Rome or Milan might already cost you that much if you buy tickets on short notice.

But maybe train tickets from Paris to Rome are not as expensive as I thought...