I get asked this now and again, and on the heels of the politics-in-the-arts post, a kind of answer.
If you go to the doctor and say, "Doc, I have a pain." chances are unless she is a Doctor Feelgood, she isn't going to say, "No problem." then give you a prescription for a painkiller based on that alone.
Differential diagnosis starts in the leaves, works its way down the small twigs and branches to the larger limbs, to the trunk and then the roots. With each response to a question, the doctor narrows the possibilities until she comes to the one she thinks is most likely the cause of your pain. Then she treats the cause, if that is possible, and not just the symptom. Sometimes she can't because the disease isn't curable, but at least she doesn't make it worse by giving you the wrong meds.
To decide on a martial art, you have to ask yourself questions. I'll frame them for you:
The first and most important question is, why do you want to learn a martial art? Self-defense? Sport? Physical, mental, or spiritual development? Social interaction? Because some arts are better at some things than others, and one size doesn't fit all.
Forty or fifty years ago, your choices were limited to a handful, and you took what you could find and made the best of it. These days, there are myriad roads to choose, and you need a map.
Start with what you want:
If you want to compete in sport, win a gold medal in the Olympics, then that narrows your choices. Judo works, taekwondo, fencing, target shooting ...
If you want self-defense, how much of your time and money and energy do you want to spend to get it? What level of defense do you feel you want or need?
Dealing with the drunk in the neighborhood pub is not the same as kicking in doors in Afghanistan waving an M4.
Martial arts, at least in the context I'm going to speak about, can be roughly broken up into wrestling or boxing; combinations thereof; and assorted weapons not limited to one's own body.
Many arts offer bits and pieces of all these, some specialize. Kyudo lets you shoot arrows; Iaido or Iaijutsu, you get to play with swords. Gun-fu needs boomware. It depends on what you want. Some can be stretched farther than others.
This is not to say that, come the burglar, you couldn't nail the guy down the hall with your longbow. It's just that the modern version of kyudo isn't really designed for that. The emphasis has been put on the spiritual. Would I want to find myself facing a kendo or iaido expert with a sword in his hands? Nope. Man who has spent time swinging long and sharp pointy things probably has the edge over one who hasn't. Especially one who likes the cutting tests.
Some arts are simple, brutal, right to the point; others graceful and beautiful to watch. Krav maga will serve you in a dust-up, and so will the combat versions of tai chi. A boxer can make do. A wrestler can, too. Which is better at the self-defense aspect? Let them argue that out.
Once you decide why you want to roll around or punch things, then you can start winnowing the possibilities. Aikido is a fun art, but if you hate tumbling, don't go there, because you have to learn how to dive and hit the ground, roll like an egg end over end and come up unharmed.
If you don't want to get into a ring in Speedos and go at it hammer and tongs at another guy who is fit, strong, and trying to take your head off, maybe MMA isn't the way to go.
If knives terrify you and you can't bring yourself to touch one outside the kitchen, then maybe you won't like the SE Asian stuff, silat, kali, arnis, etc.
Couldn't see yourself ever shooting somebody? Don't get a gun.
Every art has strengths and drawbacks and no single one can cover every contingency all the time. It's a numbers game, and you have to decide how you want to parse the chances that what you want or need will give you what you want. (And, yes, realize that there are no guarantees in life no matter what you pick ...)
So you ask yourself those journalism questions staring with the why. Then you get into the what, when, where, who, and how aspects. Once you've addressed those, using the filters you bring to the question, then you can make an informed selection.