Nike does have an advantage in a couple of different aspects of the competition. People may want to buy from them because they make a larger range of different products, it is possible to get almost anything with a Nike logo on it. They also have much higher revenue in comparison to Under Armour. As of 2014, Nike has revenue of about $27.8 billion while Under Armour is at about $3 billion. With this extra revenue, they are able to take higher risks that will lead to bigger gains. Under Armour on the other hand, doesn't have as high of revenue and therefore cannot spend as much money taking risks, leading to smaller gains.
Nike vs. Under Armour is a war that is ongoing not only in our lives (which brand should I be buying?) but also between the companies themselves. Even though Nike makes 10 times the revenue of Under Armour, they still struggle to make deals. For these large companies, it is important that they have well-known athletes, teams and coaches wearing their gear. Nike had a contract with the NBA star, Kevin Durant, and when the contract was almost up, Nike was willing to make a $200 million re-endorsement deal. Under Armour took this as their opportunity to make a better deal, to have Durant wear their gear instead. The two companies battled it out and eventually, Nike won the battle with a $350 million deal. Part of what they do is create battles so that their rivals are forced to invest more than originally planned, it is just part of the business.
So which brand is better? That can only be decided individually. It is obvious that Nike, as of today, is much more successful than Under Armour. It might be because they are slightly more affordable, or because they have better quality materials, either way, only you can truly decide which brand is the best match for you.