The most important factor for the softness of a light source is its size. A softbox will always be softer because it is a lot larger than a diffusion gel clipped to the barndoors. If the barndoors are small then you won't soften the light much with a gel, and even with large barndoors all you can do is maybe double the size.
What you do with diffusion gel on the barndoors is to make uneven light from bad par lights more even, and soften the edges of the light beam (that is you create lots of spill). As I said before, with good large barndoors, you can also make your light source a little bigger, but you will never achieve the size of a softbox.
The angle of a hairlight/highlight shouldn't be too steep, because then the light might hit the tip of the nose (which is UGLY) and you'll get a shadow on the chest.
Other than that you'll have to try and see what you like (it depends on the subject, the background, your personal taste...)
