I used to use mainly the hard pencils because they didn't break as easily then found the graphite stick really useful because they didn't. I found you can also  can get some really nice effects if you experiment with hard pencil over soft too 🙂    

 I just use a cheap pencil set, through 8b, but if I'm just piddling around i stick with the hards. I'm generally starting & stopping every 5 min, so finding the right pressure upon returning is easier