If it were me, I’d mix it up. Variety is good, so you don’t get strength and endurance in just one movement pattern.
Try a plank with your hands and feet wide, or very narrow, or hands staggered, or one foot up. If you want to keep the stress out of your low back, plank and hold firmly for 10-15 seconds, then rest 5 seconds, then repeat several times.
Try a push-up matrix. Hands narrow, neutral, wide, right hand high, left hand high, right hand low, left hand low, etc.
Consider working your back muscles, because as a society we already tend to be over-tight in our chest and over-stretched in our backs. Planks and push-ups, if this is all you do, would potentially perpetuate a tendancy towards rounded-forward posture.