Active currently beats Passive, hands down. Not just in the research per month, but in all the fun events and the bonuses that can come from it.
Passive, the "best" thing that can happen is you get the Rogue Scientist event that pushes the primitive civ up to Industrial age (minimum for Infiltration) with it possibly reach Machine age, and not only is it rare as hell (MTTH is over 400 years) there is a non-zero chance at the end of the event of the primitive regressing back to a pre-infiltration era or the Observation outpost being blown up. None of the other Passive-only events have any sort of even tangent bonus, and in fact most have bad outcomes (like the Observation outpost being destroyed, or the planet nuking itself.) You might also get smugglers, which is equally as rare, and at best earns you some minerals and energy credits.
Active, you could get the implants and abduction projects, which are a great (if early in the game, otherwise decent) society bonus, or if they are advanced enough, the chance for a free, 5 star, Spark of Genius Scientist of the primitive species. With both being much, much more common than the passive ones (MTTH for them is Six years for implants and scientist, 100 for abductions) Plus there is always the "X-COM event." Only downside is the likelyhood of pushing the primitives towards Xenophobe and/or Spiritualist, which might be something you hope to avoid depending on your own ethos and future enlightenment/infiltration/conquest plans. I guess XCOM succeeding and blowing up your Observation post would also be bad, but lets be honest, you're too busy giggling at the event to care, right?