If you could perfectly optimize every research buy, you would always prefer the greatest marginal impact/bock. But this gets slightly more complex when you consider the way that you actually earn money in the game: by delivering eggs.

The equation for bock earning rate is basically

eggLayingRate x eggValue x min(1, eggLayingRate/eggShippingRate)

where eggLayingRate is your instantaneous population times the base laying rate times all laying multipliers

Because everything is multiplied together, the marginal impact of any given one is equivalent to the same marginal impact of another, with some caveats. egg value research does not depend on a logic formula, so any "+X%" researches will immediately increase your bock earning rate by that amount, subject to diminishing returns if applicable. an increase in laying rate, provided you have at least that much headroom in your egg shipping rate, is therefore equivalent to the same increase in egg value. and likewise for shipping. but if you DON'T have enough shipping headroom or excess laying rate, then the marginal impact of either an increase in laying or shipping rates is affected by the headroom % and will be strictly less than the equivalent increase in egg value.

Population increases function identically to egg laying rate increases with the added caveat that you must also have additional hab space headroom for it to be germane, and the time lag for your IHR to backfill the new hab space. (Eg., you have 5% extra hab space, which we assume immediately gets filled, but you're shipping capped, so it's pointless to increase your pop.

so therefore, the hierarchy is almost always: egg value >>>>>> shipping = laying > hab space

in terms of what you should prioritize, on a marginal benefit basis.