That would be the Abel prize. Although I wouldn't call it a better or worse prize. The two awards simply have different goals they want to recognize. Abel is more of lifetime achievement award and Fields is more of a recognition of brilliant younger mathematicians and an encouragement to go on to greater things.
Say what?
Geez, you're right. Wikipedia:
The Medal also has an age limit: a recipient's 40th birthday must not occur before 1 January of the year in which the Fields Medal is awarded. As a result some great mathematicians have missed it by having done their best work (or having had their work recognized) too late in life.
Clearly someone needs to endow a better math prize. You know, one that is for the best math, not the best math by a specific sort of person.