What is the difference between ten and fifteen reps?
Your body does not know the difference between performing 2 reps versus 20 reps. It does, however, recognize failure. Ultimately, the goal is to perform as many reps as possible until you hit failure. That means that, yes, you could fail after 2 reps, but that means that you are throwing around some pretty heavy weight and, therefore, are at higher risk for injury. On the other side of the spectrum, you could play it safe and use 2lb weights, but you’ll be working out for hours in order to burn your muscles out.
Failure is failure. It is up to you how you get there.
There is a difference, however, in calories conserved and burned. Fat burning is dependent on time under resistance. This means that you will burn more calories when you choose a weight that allows you to perform more reps before hitting failure. It may only be 5, 10, or 15 calories, but they add up after a while.
In conclusion, focus on your goals.
If you want to burn fat, choose a weight that allows you to focus on control and hit failure at a higher rep count. If your goal is to build muscle, choose a weight that is heavy enough to hit failure at a lower rep count, but also does not diminish your form and control. Ultimately, safety, form, and control are of utmost priority no matter what exercise you are performing.