Difference between if() and if(or)? [duplicate]

I wonder that, Which of the following will tire the program more? Or they are the same?

First;

if(somestring == "somestring")
{
     if(somestringsecond == "somestringsecond")
     {
        //DO SOMETHING
     }
}

Second;

if(somestring == "somestring" && somestringsecond == "somestringsecond")
{
     //DO SOMETHING
}

Does anyone know more about tiring the program? Or some document? How can I search that?