I copy artwork all the time for students, usually during the spring and summer. I use Fuji T64, usually with bounced 500W Smith-Victor photofloods, and any problem I have ever had has been my own fault screwing up the lighting at first, not the film's. It is perfect film every time.
That being said, there is no reason that digital shouldn't work as far as getting accurate color. I shoot just as much on digital for this stuff as on film, and even when I shoot film, often I will also shoot digital while things are already set up, just so the student has a digital copy as well as a physical copy. The only reason I use film at all is 1. because schools often still want transparencies in portfolios, and 2. over all, it is cheaper and faster than digital. All I have to do is drop it off, pick it up, put the slides in a printfile page, and hand them over. With digital, I have to do all the post-shoot work myself, which is no fun at all (although you don't end up with a lot of files to work on when copying artwork, so it is pretty fast). However, I am not going to shoot film if the final product is going to be digital. Digital works fine unless you just need a simple slide in hand.
In short, I would look into why you aren't getting good results from digital, as well as exploring your film options.