Well. Planks work your core in ways crunches and legs lifts won't. I recommend all 3. I do a lot of different exercises for my core now. Planks are just ones I started working almost all the time.

Somewhere I have a biking article on back pain while your riding and all the awesome exercises to work to alleviate it. Will see if I can find it. If I do, I will photocopy for you.