I'm assuming this question is for if the animal has already got you.
Anacondas constrict their prey, if you were caught and coiled by one you would find the air being squeezed out of you breath by breath. It can take a long time to asphyxiate this way, can cause blood to pool in some parts of your body and become cut off from other parts and may even cause some of your bones to be broken, ribs especially. You would usually be unconscious by time it starts swallowing you, although it's possible to resume consciousness during the feed, upon which you would undergo a second asphyxiation. That to me sounds very horrific.
Lions ideally kill their prey by biting the back of the neck and severing the spinal cord or latching on to the windpipe and biting it closed, asphyxiating its prey. If it wanted you dead nice and quick it could certainly do it, unlike the anaconda who has no real choice about the manner in which it kills you. However, being creatures with a lot of individual variation in hunting techniques, temperaments and experiences, it might not necessarily kill you in a merciful way. The death is likely to be a lot more bloody, more open wounds, missing parts, bleeding out... Then again, drawn out deaths usually happen when it's large prey putting up a huge fight, or if the prey is down but still dangerous, the lions might feed from the non-threatening end. I think the lion would have to be just toying with the human for the human to survive long.
Wouldn't want to be killed by either, but the anaconda death sounds a lot more scary and while it's not guaranteed, there is more chance of a quick death with the lion.