Re: Measuring if/how martial arts helps one become a better person
There are two specific areas that martial arts train that can contribute to making someone a "better" person.
The first is awareness. If you are more able to differentiate between real and imagined threats, then you are less likely to act as a "threatened" person might.
The second is humility. There is always someone better. A good fraction of the time, you will "lose." If you "win," were you good, or just lucky?
Now, certainly there are aikido practitioners who demonstrate neither of these traits. But I would consider that to indicate a void in their training, rather than an indictment of the art itself.
Are the martial arts unique in this regard? I think the question is pointless. People who put a lot of effort into becoming better people (for whatever definition of the term they choose) generally succeed. People who don't, don't. So the best practice is the one that can hold you interest for years or decades.