There’s really no better way to conserve animals, plants, etc. because the end goal of conservation is to keep populations healthy and going. Both have their advantages and disadvantages.
For example, as the previous answer points out, in-situ conservation keeps animals and plants in their natural habitat, but ex-situ conservation takes them out of their natural habitats. This difference can breed animals to survive in the new habitat instead of completely conserving the original species, but it would take many, many generations for that to happen.
However, in-situ conservation is probably the more preferred method by conservation experts. It keeps everything in the natural habitat it was born in, and it keeps habitats from completely changing because animals have moved around and new food chains were established.
I don’t think it really matters which conservation method, in-situ or ex-situ, is more efficient, because clearly, both methods are needed. The ultimate goal for conservation is to protect plant and animal species and the ecosystems on which they depend.
Sometimes this means using ex-situ methods and sometimes it means using in-situ methods. Both methods have their advantages and disadvantages and costs. If cost was not a consideration, I would rather see in-situ conservation methods used because these methods leave the plants and animals in their natural environment as opposed to an artificial environment like a zoo or aquarium.