Since you are laying the laminate yourself, cutting the sink hole first seems to make sense. You should be able to get a cleaner cut with the laminate trimmer. But I would be afraid of using MDF around water in a self built countertop. If you don’t get it sealed properly, it will swell up and destroy itself with just a little water. I remodeled my kitchen and used this concrete kit from z counterform. It was fairly easy and makes a solid and practically indestructible counter.