replace both springs. Over time springs loose their resilency (springiness) Replacing the one that is cracked will get you back on the road for minimal cost but the spring on the other side may be settled a bit and lost some of its resilency so the car may sit an almost un-noticable ammount lower and ride slightly different on the side of the old spring.
Also, whats not to say that the spring that is not cracked right now has a large ammount of work hardening in the metal which will lead to it cracking wether it be next month or 50k miles from now. Sooner or later the other spring will crack.
