For the record, I train in Taekwondo, Hapkido, and Brazilian Jiu-Jitsu. I also have training in Japanese Jiu-Jitsu, Karate, Judo, Kickboxing, Krav Maga, Filipino Martial Arts and Shaolin Kung Fu. Am I great in every one of these arts? Definitely not. In most, I have only a basic understanding. In others I am very knowlegable. If I were in a fight and had to use these, which would I use? In that case, my list gets fairly small.
So let's examine this from both points of view. I know instructors that feel that their martial art is the best and that is all you need. Other instructors, such as myself, understand that every art has a weakness, so to train in arts that flow together in multiple zones, is far more effective. So which of us is right?
Although there might be many answers to that question, I believe that the answer falls somewhere in between. Everyone should have a martial art that they feel very comfortable with and be trained to use without thought. But, also having knowledge of other arts can aid a martial artists in unexpected situations.
For example, let's say that a Taekwondo student is defending themselves against a wrestler. Taekwondo is very effective at kicking range, moderately effective at punching range, becomes far less effective at close range, and is 'almost' useless during a takedown, or on the ground. The wrestler is ineffective at kicking range, not much better at punching range, but very effective at close contact, takedown, and ground fighting. So, who would win this fight? If the Taekwondo student could maintain the range and effectively use kicks, without being grabbed, the Taekwondo student has the definite advantage. If the wrestler gets inside the kicks and shoots in for a takedown, the wrestler has the advantage. The winner of the fight would be the student who held their advantage the longest while minimizing time spent in their weak zone.
So which martial art is better? To me that is a simple answer. The answer is whatever art you train in. All arts have strengths, and all arts have weaknesses. If you train hard enough at the strengths, you can overcome the weaknesses. So is it better to train at one art or many? Again, that is your choice. No martial artists wants to be a Jack of all Trades, and Master of None. But, that also doesn't mean that you should master on art and then stop learning.
The perfect martial artists is a perpetual student. I train in Taekwondo, every day. But, I still train in the other arts to expand my knowledge. If I ever again have to defend myself, I would focus on maintaining distance control, but if things don't go well, I am also fairly competent on the ground. Everyone should have a plan, then be ready to change that plan, almost immediately. My old Taekondo instructor used to talk about everyone having a toolbox. The more tools you have the more options you have. My Jiu-Jitsu instructor also talks about having a game plan. You should focus and train your "go-to" moves regularly, but also continually train additional moves to expand your knowledge.
So, to sum up the above. The answer is simple. Just train! It doesn't matter if it is one art or several. They are all effective against an untrained aggressor and someone properly trained in martial arts generally has enough self-control to avoid a fight. Once training becomes a life style, you quickly learn that there are always more lessons to be learned. Be open, because you never know what you might learn or when you might need it.