For fun, I looked back over some past characters to see where they fell in terms of exploration vs. observation. If I recall I did minimal "gaming" of either. All of these were long-lived.

Exploration: 16k, 21k, 17k, 17k, 16k, 12k, 20k.
Observation: 37k, 24k, 19k.

The rest were too old for observation to be listed in the PBF. Of the three that had both listed, one was roughly equal in terms of one vs. the other, and the other two both had more observation XP than exploration.

So it looks for these characters I would have maxed observation but left exploration-derved EP on the table with each of them.

I've said this before, but my personal feeling is that the caps should be reached by roughly ~90% of long-lived characters who are exploring / observing at a "reasonable" level. The alternative is to encourage people to explore / observe at an "unreasonable" level or risk leaving EP on the table.

With that goal in mind I'd probably cap both at 20k.