Those rides are essentially the same.

Why the hell would one do a 100 km TT "group" ride in the first place??

The answer is the guy that bags the 100k tt, then actually rides with the group, hammering like most groups do, until there is a lone break at the end sprinting for the last city limit sign. That guy also had the most fun....

Well let’s clarify to tt pace line not a group. There is no side by side. But there are some strong players.
Higher NP slightly lower AP

100km in a group with NP 215 means you're probably going faster than someone solo doing NP203. It could make quite the difference in terms of duration of the ride. The intensities are really not very different, but depending on the speed difference the individual may be riding significantly longer which could result in a higher TSS and hence, training effort.

Edit - such a small difference in NP would make it not a very hard group ride. The group ride I do have much higher VI's... differences between AP and NP are typically 60~70W different (vs 20W in your example).

Agree re: np and ap differences, and I will see 60 watt differences in road racing. Team time trial riding with friends the np usually ends up higher but by about 20 to 30 watts only, ap down a smidge 5 or 10 at most. Duration Usually made the same, often keep going alone to make up time or distance to account for higher group pace

Yup, that's been my experience, group roadie rides or races have a much bigger variance due to bigger group sizes and lots of surges to attack or cover gaps. TTTs are steadier, you don't want any power spikes or big accelerations. Did a roadie ride this morning where there was a 54W difference between AP and NP. Last TTT I did the difference was only 22W, though that is probably lower than normal as we only had a 3 man team and one of the guys was having a bad day so I probably spent ~40% of the ride on the front. Also a lot of wind and I was the tallest rider on the team so felt like I never got much of a break!

To your OP, if you normalised for time instead of distance (e.g. Both rides were 2 hours), then the ride with the higher NP is a tougher and more beneficial workout. Physically at least - mentally you do also need to log at least a few of those steady solo efforts as well to be ready for race day.