Trader A only trades one instrument/pair and gains 10%.
Trader B trades multiple instruments/pairs and gains 10%.
Argument for Trader A:
Trader A only needs 1 instrument to get that 10%.
Argument for Trader B:
Trader B is a versatile trader and is likely to be less affected should any market condition changes.
Trader B - your argument makes it quite clear why.
Thank you for your opinion. Appreciated.
But Bruce Lee said…
So maybe he would prefer Trader A? 
Yet Bruce Lee created Jeet Kun Do. A style mixed of different styles. He wanted what was useful for him, from each style. He wanted to be flexible in his approach.
Trading one pair is good for some people. Some people prefer to be diversified.
There is no better trader. It depends on what each trader wants.
We all have different personalities, lifestyles, and goals. We all have different ways of achieving internal peace.
Perhaps, you could be more specific. What do you mean by ¨better¨?
I’m asking what would be your definition… 
I would say trader B over long term would win due to diversification.
Because diversification is a very important factor in risk management.
Also for argument sake, is instrument your skills aka your edge? Also something to think about…
“A jack of all trades is a master of none, but oftentimes better than a master of one.”
Trader A for me.
Nothing beats focus, pick one strategy, and pair and run with it. Rather than scatter your attention across multiple strategies, pairs and market movements.