Hi there,
Since you're just visiting Tokyo and Osaka, it probably won't benefit you to buy a pass. You're better off buying the single train ticket, unless you plan to make any day trips or short stops on the way to or around Osaka. In that case we would recommend the Arch Pass. Also, if you're going to Osaka then back to Tokyo it would make sense to buy the Arch Pass.
Please note that the Arch Pass is only valid for 7 days.
With an Arch Pass, it would take 5-6 hours to get from Tokyo Station to Osaka. The Tokaido Shinkansen will take about 2.5-3 hours.
Hope this helps!