I do both, but Zazzle is much better. Art of Business Cards generates income but not like Zazzle. I use to upload a lot to ABC, but then I got on Zazzle and the money is so much better. I haven't uploaded to ABC in over a year but I get sales there every month.
The thing about ABC is you have to save the files as tif before you can upload. Then you have to choose from their layout that best works with the card and put a description and category. Their card size is different than Zazzle so it's hard to do the same design for both. I would upload more if they were the same size.