& get_instance() vs $this

When should we use $this and when & get_instance()?I have a library file .The code is
class Test {

public function testf()
{
    echo 1;
}
}
Now I load the library in my controller function
$this->load->library('test');

Now in my view if we write $this->test->testf();.We get result 1;
Now the fact is I load the library in view file.Then it give error Undefined property: CI_Loader.
if we create an instance in view then its woking properly.like 
$ci=& get_instance();
$ci->loadl->library('test');
$ci->test->testf();
then it gives result 1;
My question is where to use $this and where to & get_instance();