Could be either of the above.
When were the calipers last serviced? If it's over a year I would suggest you strip, clean and inspect them. It's likely that the inside pad is stuck, so you aren't getting anything like the amount of braking force you should.
If the discs/pads are nearing the end of their life (>50% worn), I would replace them at the same time as servicing the caliper, they aren't expensive. Makes sense to start with fresh so you can rule out the discs and pads.
Do this, and your brakes will be fine for another 12 months minimum. It's best to strip and clean them at least once a year.