First of all it’s not a black and white decision so don’t worry if not everything is clear for you.
There are multiple factors deciding which is the right choice in my opinion : most importantly, where are you travelling? Eurail lose its value in Spain where seat reservations are everywhere and they’re not always available online. On the other hand Eurail is excellent value in Switzerland where tickets are expensive and seat reservations aren’t a thing (= full flexibility). Train travel isn’t exactly inexpensive and Eurail is a good way to keep prices down.
Advance tickets might be cheaper but you’ll be tied to a specific connection. Eurail definitely gives you more flexibility.
The price difference between a 1st class and 2nd class pass is also smaller than with tickets.
All that to say : Eurail wins for me, especially for a 1st class trip. There is also a 15% sale at the moment so enjoy that offer!
---
Now some (slightly chronological) advice for your trip :
Direct high-speed trains between Paris and Switzerland have a 29€ reservation fee (39€ 1st class). To save money take any high-speed train until the border (somewhere like Mulhouse or Strasbourg) : 10€ limited fare, then 20€. It takes about 30 min longer. Have a look at this general guide : https://www.seat61.com/interrail-and-eurail-reservations.htm
Where are you going in Switzerland? The pass gives you a 25-50% discount on multiple mountain peaks and lakes. Boats on Lake Thun and Brienz are fully included : you have access to the upper deck with a 1st class pass.
Do not miss the Albula & Bernina railways between Switzerland and Italy. It’s a truly stunning route and quite a convenient route to Lake Como too. More here : https://www.seat61.com/trains-and-routes/bernina-express.htm
You don’t have to pay 36 CHF extra for the panorama carriages with big windows. Simply board the regular unit at the front : there’s a small cozy 1st class area and you may see the driver’s cab if the blind isn’t shut. There are pull-down windows for a cool experience too.
Seat reservations are required on fast/high-speed trains in Italy. Availability on the day or the day before is rarely an issue as trains are more frequent than in France or Spain (= you remain flexible).
It might make sense to get a 7 day pass, depending on which day trips you’re planning in Switzerland. Feel free to ask further questions.
Prices, especially long distance, depend on a number of factors, such as distance, how much time you book in advance and country. Switzerland can be pretty expensive if you buy single tickets and the pass is very good value there. International TGVs can also be very expensive.