While I can't say much about the snaps mentioned, I can tell you that the magnetized closures I use do not affect credit cards. Sometimes they will affect hotel room key cards.
http://www.hakidd.com/product.php?iproductid=BU5630K
The link is the kind that I use a lot of the time. If I feel I need a stronger hold, I sometimes double up (use two layers per side vs one)