Your test case isn’t very helpful. push_back takes a container element and copies/moves it into the container. emplace_back takes arbitrary arguments and constructs from those a new container element. But if you pass a single argument that’s already of element type to emplace_back, you’ll just use the copy/move constructor anyway.
Here’s a better comparison:
Foo x; Bar y; Zip z;
v.push_back(T(x, y, z)); // make temporary, push it back
v.emplace_back(x, y, z); // no temporary, directly construct T(x, y, z) in place
The key difference, however, is that emplace_back performs explicit conversions:
std::vector<std::unique_ptr<Foo>> v;
v.emplace_back(new Foo(1, 'x', true)); // constructor is explicit!
This example will be mildly contrived in the future, when you should say v.push_back(std::make_unique<Foo>(1, 'x', true)). However, other constructions are very nice with emplace, too:
std::vector<std::thread> threads;
threads.emplace_back(do_work, 10, "foo"); // call do_work(10, "foo")
threads.emplace_back(&Foo::g, x, 20, false); // call x.g(20, false)