Academically, it is in your best interest to report it. If the class is curved, the cheaters are not only unfairly outshining you, but they are unfairly lowering your grade. Even if it's not curved, unless it's an enormous class, the professor's perceptions of who did well may still be influenced by who did how well. If a student that the professor expects to score low (based on their in-class participation) instead cheats and scores high, the professor may decide that the exam was easier than he originally thought, and value your honestly earned mark less.
In the long term, if cheating is rampant in your school, this will soon become known. The value of your degree will drop even if you didn't cheat, because how can I know you didn't get your degree by being one of the infamous cheaters, and managing to avoid getting caught?
In a class where all exams are multiple choice and grading is completely objective and not curved, cheaters have no effect on you (except for the long term consequences stated above). Only then you could say that pragmatically, neither reporting nor not reporting helps or hurts you appreciably, so you might as well not bother.
But then there is also the ethical aspect. Cheating is bad, you are expected to not cheat and report cheaters by the instructor and school administration, and you probably even signed agreements and made honor pledges to this effect. So, it would be dishonest for you not to report it - not reporting isn't even a valid choice, it would be a dereliction of your contractual and ethical obligations. In practice, you will never "get caught" and be punished for failing to report cheating - but whether you get caught is immaterial to ethics.
So, speaking in terms of your credentials in school and beyond, there is absolutely no reason not to report it, and strong reasons against not reporting (eg. you promised in writing that you would report when you enrolled). It would be extremely unusual for a professor to somehow punish you for reporting.
But that's not the whole story: Like it or not, the people who you reported will hate you for it. They will tell their friends to hate you for it. If they are popular, you will quickly become very unpopular. Not all your classmates may have the same concept of integrity, and some may hate you for "siding with the establishment and betraying your comrades" (as they see it).
Your classmates may one day end up being your colleagues. If you get a reputation as a "rat" who has dubious allegiances, and cannot be relied on to have his friends' back (even though the cheaters are probably not your friends, they are only united with you in their struggle to get good grades) against a perceivedly antagonistic and unfair institution, it may become difficult for you to be seen as trustworthy.
Consider how in history there have been oppressive, unjust regimes which employed "informants" to report on people who try to circumvent or oppose the oppression. Clearly, this is not the same as reporting cheating: For one, academic cheating policies are clearly just, ethical and reasonable (unlike oppressive regimes). But the point is that following a rule is not automatically a just action. It is hard to definitively say what is just and what is unjust, so a lack of skepticism towards even apparently just rules is taken by some as evidence of inability to reject rules even when they are unjust, and generally lack of critical thinking ability.
To answer your question, you must ask yourself: Are you an idealist, or a pragmatist? If an idealist, there is no question that you should report the cheating. But if a pragmatist, unfortunately, it depends. You must further ask, which do you value more: Your reputation among your peers, or your formal academic credentials?