I think it depends on what you want to use it for. The middle slot of a flat cardholder can hold a ton of cards. If you have mostly cards then go for the flat cardholder. If you use coins and cash a lot then go for the zippered one.
I have the flat cardholder from Chanel and I really like it for when I need to switch to a mini bag and don't need to carry around cash. It's super versatile, sleek, and holds a lot of cards.
I also have a flat cardholder with a zipper (similar to the Chanel one in design but not from Chanel), and I only use it when I travel and need to carry some cash and coins around, and only the basic cards like my ID, and two credit cards.