Let me take a purely spiritual perspective which I believe to be true.
Whatever we take from the universe, we have to repay it back equivalently - this is the immutable Karmic Law.
All in all, when we consume external energy as food to satisfy the energy needs of our own body, we have to repay that energy debt back to universe in one form or another. This debt is largely mitigated by the money we pay to buy both vegetables and meat. It is the money that we earn through our hard work. We spend energy in doing the hard work. So this energy exchange helps balance the Karmic law of “what you take is what you have to give back”. This is also the reason why we should always thank the food before we eat it. It mitigates our debt further.
We suffer when we have a heavy Karmic debt to repay. We achieve happiness only when our Karmic debts are low. We achieve the Divine when all Karmic debts are paid off. If all one is interested in is to live the worldly life, accepting all its compromises, its impermanence, its illusions and its traps, then one can define one’s comfort zone and be temporarily happy in it, no matter what one eats. You can find such examples everywhere in the world. However, if one wants to evolve spiritually as well, then one has to be conscious of any action that adds to one’s Karmic debt. Again, to achieve happiness, Karmic debt has to be lowered.
Everything is conscious in this universe including both plants and animals. However, plants are less advanced life forms than animals. It is also the Karmic Law that the more advanced form of life one kills, the more debt one accumulates. This law is somewhat reflected in our human laws as well. If someone kills a human being, one is judged differently than if one kills a deer. This principle can also be understood in terms of energy. Animals have higher conscious energy than plants. They feel more pain and suffering when killed than plants do. In fact, even when an animal is killed "humanely", it starts feeling that the end is near. It then secretes emotions of fear and anger into its energy body. These emotions of fear and anger are then transferred to the one who consumes this energy as food.
Deeper teachings of spirituality also mention that one should not take from the universe that which one cannot give back. We cannot give life back to an animal but we can definitely sow a new seed to grow a plant. Now, there have been saints who would not eat even a fruit until it was ripe and fell from the tree naturally! Our current world is not suited for such extreme austerity. So a middle path should be followed if spiritual progress is desired.
Being vegetarian keeps one’s Karmic debt as low as practically possible. Hence, it helps in spiritual progress because in a world where one doesn't even get five minutes to devote to one's spiritual practice, it is advisable that one should at least try to keep one’s Karmic debt as low as possible. If you are not earning much, the only way to not accumulate more debt is by spending less. Few inconveniences caused by practicing vegetarianism bring significant savings in terms of one’s Karmic expenses.
However, to start a spiritual practice it is not necessary to become a vegetarian. Vegetarianism comes naturally. There is no need to force oneself to be a vegetarian. If you feel from inside, then by all means you should practice vegetarianism. In addition to vegetarianism, it is important to develop intensity and sincerity in practice. Spirituality is about developing a close relationship with the powerhouse called the Divine to the extent that you start having an inner dialogue with the Divine every moment of your life. A person who eats meat, or who drinks (alcohol), can also establish a close relationship with the Divine if he or she has enough intensity. For example, there have been realized masters who used to eat fish. However, for an average person who doesn’t have the blessings of a realized master to develop such intensity is a rare achievement.
Also, to be able to practice any kind of spirituality, it is important that we remain alive and mentally healthy. Therefore, if one's survival is dependent on eating meat, one should exercise that only option. If you are born as an Alaskan Eskimo, then it will be hard for you to survive as a vegetarian. Understand that the Spiritual field is supremely conscious and infinitely intelligent. So it understands our circumstances and presents to us many paths to enable us to realize our oneness with the Divine and achieve eternal happiness.
Only in some cases, when one becomes self-realized, one becomes so aloof from the material world that what one eats doesn't matter. I once read the story of a Spiritual master who when he went to a surrendered seeker's house ate meat much to the chagrin of his initiates who were always asked to be vegetarian! When the zealous initiates found a chance that evening, as the group sat around the bonfire, they expressed their disappointment to their Master. The Master didn't say anything. He just reached out to the red hot coals in the fire, held some in his hand and gulped them like they were jelly beans. The initiates understood his answer.
One should eat healthy as per one’s circumstances and nature. Make efforts to be a vegetarian as it will reduce your Karmic debt. Above all, always have the love of the Divine in your heart. See everything as the gift of the Divine but do not desire the gifts more than the Divine Himself.