Biros actually feel quite a lot like using a pencil, minus the possibility to erase of course. You get dark or light lines depending on how hard you press and it's quite easy to fall for the temptation to correct mistakes by going over the same area again and again... usually resulting in nothing more than a messy nest of lines and no real improvement.
That's why I love using a more fluid ink pen like the G-Tec when I'm out and about with my sketchbook. Its thin tip is precise and it forces you to be decisive when you put down a line. If the line is wrong you just leave it and move on which makes sketching fun and liberating and there's a definite charm to describing things with as few lines as possible.
With the risk of this turning into one of those "this is the tool to use" talks, I'd just like to end on the note that of course it's all about what you're comfortable with. It's worth highlighting the benefits of using a tool that makes you take responsibility for your lines and preferably think before you put something down however.