What you can do on the machine won't be close to what you'll press with the barbell. Machines work you in 1 plane of resistance, whereas free weights take three. Machines tend to take the stabilizers and synergists out of the equation that are vitally important with free weights.
My advice-start benching both with the barbell and dumbbells. Forget about going to failure and keep 1-2 reps in the hole.