Which is very difficult? CE marking or UL?
That is a difficult question to answer. It can vary from one device to the next, plus it really comes down to your design and how well you prepared based upon the requirements of the standard. In some instances the CE mark can be self declared, whereas UL Listing requires examination of the product by the independent laboratory. If you plan on selling the product in the US, The CE marking certification declares that the product complies with European standards for health, safety and environmental protection.