Depends on your preference!

What other routes are you taking besides those longer Vienna-Venice, Nice-Cologne?  And when are you travelling? 

If you travel solo and have the budget for it and like your rest and personal space, a bit of extra service on some routes, 1st class is your go to.

Quality of first class can be very different sometimes. But on long distances, it's usually quite nice. 

If you like to engage in conversation with random people, like people watching you might want to stick to second. If you avoid public holidays and peak hours, it's also pleasant to travel.

Upgrading for individual trips using a 2nd class pass is often not possible and rarely financially sensible.

If you want to travel 1st class then buy a 1st class pass.

Can I ask why you want 1st class only for these 2 trips out of 15 days of pass use?

If you can afford it go 1st. It is personal preference but speaking from experience it is worth it in Western Europe 

Thank you I have 10 trips in total but spread over 15 travel days just thought as these two are basically all day it might be worth upgrading to first class however I do love talking to people so maybe that’s not the best option in first many thanks for all your responses it is so welcoming.

Just compare it to eco and bisnis in a plane. In fact in german ICE the seats are the very same-but with nicer cloth on it and more space around. Mostly bisnispeople who often sniff at these unkempt tourists which they know to hold these passes ´for so little money whereas we pay such a lot´. DO note that passholders do NOT get entry to lounges in DE (yes in FR, IF having a RES for a TGV next hrs, but these are hardly worth the trouble to even find them).

Köln Nice is a rather long trip and also not that str8 forward. DO avoid the probably shown first combo of Thalys to Paris, then SNCF-TGV-cost the earth and brings only trouble-also change stations in P by metro. Better to use the route via Swiss-and break up somewhere=many more nice views and ´OH whow nice!”screams. Except when you as kiwi already live in the snow paradise on south-isld.

Many thanks