You don't have to implement a look and feel. You just apply it to your Swing UI. The code is the same for both.
It is also a matter of taste which one renders the best interface. They both should work fine they just (as the name says) look and feel differently. The differences in the look you can see yourself. Don't know about the differences in the feel (behaviour) and I doubt it's possible to give a satisfying list.
Just implement your UI the way you want it and try out different look and feels afterwards using UIManager.setLookAndFeel()
.
Personally I think it's a good idea to use the system look and feel. It makes the UI look like the system default. E.g. on Windows it looks like a Windows program and on Gnome like a GTK application.
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
You can read further about it at http://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html