Tough call indeed. In fact, you’re really going to have to try both before you make up your mind. Still, yoiu do ask so for me, it has to be Zermatt. Val has fantastic skiing, wonderful scenery, some great restaurants etc. However Zermatt just has a unbeatable ambience, absolutely mindblowing scenery (not just the mighty Matterhorn) and the most fantastic restaurants (on and off the mountains) in the Alps.
If your question is purely about skiing, it probably depends on what kind of skier you are. If it is all about piste kilometres for you, then Val is probably the more logical choice. If it is all about tough runs, both places will press all the right buttons. Ski convenience - Val comes out on top - but convenience is unlikely to be the reason for picking either resort.
Good luck trying to make up your mind. Maybe flipping a coin is your best option!
JJ