I agree with the above post, they must have experience with cover-ups, but its also important that they can emulate the style and final results you want. I was in the same dilemma when deciding on who should do my sleeve (it wasn't a cover-up). I was torn between two artists, one who did amazing portraits and the other who excelled in sleeve work but was not as detailed. I decided that I would sacrifice abit of detail in order to have a well put together sleeve, and I couldn't be happier. In short, find out whats important to you the most regarding the final results you want to see and then go with the Artist that you think can make it happen the best!