According to the Harvard Medical School, if you want a stronger core, you should skip the sit-ups and opt for planks!
Why? Sit-ups can be hard on the back as they push your spine into the floor. Sit-ups work your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When hip flexors are too strong or too tight, they tug on the lower spine which can be a source of lower back discomfort.
Second, planks recruit a better balance of muscles on the front, sides, and back of the body during exercise than sit-ups, which target just a few muscles. Remember, your core goes beyond your abdominal muscles.
Finally, activities of daily living, as well as sports and recreational activities, call on your muscles to work together, not in isolation. Sit-ups or crunches strengthen just a few muscle groups. Through dynamic patterns of movement, a good core workout helps strengthen the entire set of core muscles — the muscles you rely on for daily activities as well as for sports and recreational activities.
Source: Harvard Medical School