An Up Cut router bit cuts upwards as it rotates and cuts. So it results in a clean finish at the bottom of the material. But this same cut direction results in a rougher finish on the top of the material. A Down Cut bit’s blades are at the opposite angle, so it cuts downwards as it rotates. So the top surface of the material is the side with the better finish, and the bottom surface is rougher. Your choice comes down to which side you want to have a better finish. The ultimate is a Compression bit that combines Up Cut and Down Cut to leave smooth surfaces on both sides. They are quite expensive, but you may feel it is worth the investment.