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The soul of an individual is made up of two parts: The ghost, which is eternal and lives on after your death, and mana, which is the life energy that keeps your body functioning and your soul grounded to the mortal realm. When a person dies, their ghost travels to the hereafter while their life energy dissipates into the background of the world. All people are born with a set amount of mana, and there is a process in which one can steal theirs and add it to their own.
This has led to human trafficking rings developing around the world in which people are kidnapped and sold to individuals interested in this practice.

For this procedure to work, ancient runes must be carved into the flesh of a living victim from head to toe. Once this is done, the person is killed in a sacrificial ritual. This allows the ghost to leave the body, but keeps the mana contained within the corpse. From here, the witch/wizard can do one of two things. They can seal the reservoir of mana into a containment jar(s) for later use, or absorb the mana themselves.

Sealing the mana in these specialized jars for later provide a temporary boost in power for spells or rituals. However, a significant amount of mana is lost upon the transference process from the corpse into the containers. It is also hard to store, as the mana eventually erodes the containers and decreases in quality over time. Absorbing it into your body immediately adds a permanent boost to your supply and makes your magic much stronger and can even extend your lifespan. Even though people are born with a set amount of life energy, the body can hold an unlimited capacity. All of the victim's mana is absorbed upon this process, with none of it wasting.

Why would the 1st option be more attractive to mages if it provides less returns on the investment?

Why do most of us not use stolen money to increase our bank accounts?

The simple reason is that some indeed do it. However, most people (including a lot of other mages) tend to look with disfavor on those who go around killing people to steal their mana (or money), and band together to imprison or kill them. Therefore, mana-stealing is risky. While it can increase your power & extend your lifespan if you manage not to get caught, if you do get caught both are dramatically shortened.

Why would the 1st option be more attractive to mages if it provides less returns on the investment?

Simply put, you don't. Well, you don't rationally do it. You might do it if absorbing someone else's life energy is taboo -- you know, more taboo than killing them and drinking their corpse.

Rationally speaking, mages will not take the path which returns less on their investment because rational people in general don't do that.

So to find an answer to your question, we first have to free ourselves from that choice of wording. You have some implied concept of what an ROI for mana is. Based on that formula, you see mages will only choose to absorb the mana. Thus, we need to identify a different formula for ROI that someone might use.

Let's create a hypothetical example. This example is intentionally extreme, to prove a point. You have a choice. You can either have 500J of energy, right now, to use or dissipate at your leasure, or 50J/day of energy. Let's say the 50J/day of energy comes in the form of a solar panel, so every day you get to have 50J more power to do your bidding. Over 10 years, that's 182,500J. Clearly the long term ROI is much higher if I take the permanent increase in power rather than the one-shot increase in energy. Only a fool would take the energy.

Now lets make this hypothetical situation dark. Really dark. Someone has broken into your house. They have beaten you. They've made it clear they're about to kill you, but not until after they finish raping your wife. Now let's say the 500J of energy you could have right now is in the form of chemical energy. It's the chemical energy in the nitrocellulose stored behind a 9x19mm Parabellum, chambered in a gun (which happens to be roughly 500J, depending on the manufacturer).

Your other option is a nice solar panel. Useful on sunny days.

If your mana container stores mana in a way which can operate differently than garden-variety living mana, it can be valued fundamentally differently than the living mana.

You can come up with any number of ways this could be different. Just to throw out one, what if you don't always have full control over how much mana a spell uses? The spell uses enough, however much that is. Dangerous spells might come with a high risk of simply consuming all of your living mana, freeing your ghost to depart. This might indeed be a check on the kinds of spells mages choose to cast. If you drew the mana from a container, it isn't necessarily connected to your living mana. So if the spell sucks the container dry, you stay alive.

This particular variant would make the creation of containers even more taboo than the, you know, killing people and raping their soul for mana. Not only would you be doing such things, but you would also be doing these things inefficiently, simply so that you could do dangerous spells without having the proper skill to manage their mana consumption. Particularly vile characters might keep entire storerooms of such containers on hand, requiring a substantial stream of victims to keep the stores topped off as they decay.

Enslaved mana.

The problem with a human slave is that your slave is coerced through fear. A slave might see it in his or her self interest to obey because of a lack of better options. If a better option presents itself (e.g. run away, kill you and run away, not do dangerous job) the slave will take it. That will leave you in the lurch if you are counting on the slave.

A person who enters into a contract and will be paid might be more reliable, because he sees that a similar contract in the future could be useful. Employees are mercenaries - better than slaves because voluntary, but still less than completely reliable.

Better yet are workers / soldiers who are loyal to you because of training, acculturation or family ties. The best is your own self.

You could make mana fungible, like coins or oil. Or you could declare stolen mana is in essence enslaved mana. It may not behave as predictably as your own mana - or not predictably at all. You might want to reserve using this mana for fairly risk free / low importance magical endeavors. Just as your slave soldiers might decline to fight, or desert, or turn on you the slave mana might behave unpredictably. You could have the behavior of the slave mana depend on the individual who is its source. The magical parallel is the summoning and binding of a demon - a potentially powerful tool but one who will incompletely obey or betray you if it sees advantage and gets the opportunity.

A person needing more mana for some important endeavor might turn in desperation to slave mana - and the results could add interest and energy to your narrative.