If these are the only traintrips you intend to make, then a pass is not worthwhile-it will only be-perhaps-if you travel longer trips on at least 4 days in EURope.

IF you decide to get a pass, then you must need for these trains ALSO a Seat (or sleeper if so wanted) REServation-which is not easy to do online, easiest do that simply on arrival at those cities-you pass the station anyway then. THis for the normal case where you plan to stay a few days, see town, and move on. That shoulld be no rocket science. If you decide on the daytime trains, then you do not even need that on these routes. This is to prevent overcrowding-but the eternal fear of many newbees here that trains are solidly booked weeks ahead is complete nonsense. A seat-RES cost around 4-5€ and thus will be clear to not cover the trip as such. (booked fixed advance from 29).

A Eurail pass is by no means mandatory in order to travel on trains in Europe. It's simply a travel product that operates as a single train pass that lets you travel as much as you want on participating European train networks for a set amount of travel days. For high-speed and night trains an additional reservation fee is required. In many cases, such as a single journey, this is often not the most economic way of traveling. In that case, you can simply buy a full fare single ticket from the relevant train carrier. You should be able to purchase full fare tickets for both journeys from OBB's Nightjet.