Do we need the Swiss Saver Pass, the two country Eurail Pass, or just point to point tickets?
Well, we will just try to let you know the cheapest point to point options, so that you can afterward compare it with pass fares and make your choice.
We fly into Paris and will catch a train to Luzern, Switzerland the same day.
Since there is no direct train to Luzern from Paris, thus you will need to make at least one change. Most convenient station to change probably would be Basel. So you can either purchase Paris-Basel ticket (discounted tickets start from 20 EUR (tgv-europe.com), but you need to buy them in advance) and then another Swiss domestic Basel-Luzern (31 CHF full fare or from 12.4 CHF discounted SuperSaver tickets might be available as well). Or on the contrary you might hunt for one ticket Paris-Luzern presupposing change in Basel, those might be found for as low as 40.8 EUR.
Similar ticketing for your way back.
While in Luzern we would like to do the Golden Roundtrip to Mt. Pilatus one day
Neither Swiss Pass or Eurail gives you free travel here, both give only 50% discount, so instead of 66 CHF you'll pay 33 CHF. Similar story with Jungfraujoch railway (if you plan to go all the way to the top)
and another day take a train to Interlaken.
Standard full fare tickets cost 30 CHF, discounted from 12 CHF
we will possibly take the train to Gruyere, etc.
Full ticket cost 58 CHF, discounted can be purchased only till Romont (from 31.2 CHF), full fare Romont-Gruyeres 12.2 CHF
Approximately 3 of these days we will be taking the train for day trips outside of Paris.
Here you just need to go to tgv-europe.com and search for full fare or possible discounted fare Paris-destination of your choice-Paris. Then just sum it up and compare all of the above to the passes you had in mind.
Resume: in general pass allows you certain amount of freedom while traveling, but if you have an exact plan with fixed dates, then you can beat pass price by purchasing in advance discounted point to point tickets (most of which are non refundable).