Journey time and train types
The fastest service, the Nozomi, covers Shin-Osaka to Tokyo in roughly two and a half hours; the Hikari and slower Kodama services take longer but stop at more stations along the way and are sometimes covered more favourably by rail passes. Trains run extremely frequently through the day, often multiple times an hour, so building in a fixed departure time rarely feels restrictive.
Booking and seating
Reserved seats are available and worth it during peak travel periods, though unreserved cars exist on every train and are usually fine outside the busiest windows. Tickets can be bought at station machines, ticket offices, or in advance online, and the process is straightforward even without Japanese.
Flying into one city, out of the other
If your trip covers both cities, flying into Tokyo and out of Kansai Airport (or vice versa) avoids backtracking to the same airport and is a common, efficient way to structure a two-city Japan itinerary — worth weighing against the cost and convenience of simply shinkansen-ing both directions from one base airport.