Question in title.....

leg raises are good for your torso and back
if you work your abdominals with out working your back muscles you can harm your back so remember to work your back.

I find situps much too easy, so I would say that leg raises are probably better for a strong abdominal.

Leg raises work on your lower abs, situps upper abs, do them both if you can.

Leg raises work on your lower abs, situps upper abs, do them both if you can.

Quoted for the truth. Reps

If you want a exercises that is simple and kills your core. Do jacknives on a swiss ball. Requires core strength, and stability.

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/2y03dkaK1O0&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/2y03dkaK1O0&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>