It really depends on what you’re trying to accomplish.
If, for example, you want to allow your players to add custom cards to the game, the easiest would probably be to use ASCII files since regular, non-tech-savvy users can open them with virtually any editor (like Notepad++) and change them. Simplest form of modding, if you will. Take a look at Starbound, that is precisely how you add new items, tiles and codexes to the game.
If, however, you have no need for that and are pretty much a one-man-army working on the project Nick Darnell’s answer over there sounds like your best bet.
But if you, for example, want to display the cards on a website it might be wisest to shift your card data to an external, cross-platform compatible third party program like MySQL. Sure, an Excel Spreadsheet would work fine for that, too, but in most cases you’ll have to write your own parser. Chances are you’ll find an SQL interface for any platform you’re using.
Plus it gets complicated if more than one person at a time attempts to alter card data on the server: at a point in time t0 two individuals check out the spreadsheet at version 1.0. At t1 person A checks in version 1.1a of the spreadsheet. At t2 person B checks in version 1.1b of the spreadsheet and all the data added to v1.1a is lost.
Summa summarum: every approach has its pros and cons and it is up to you to decide whether the pros outweigh the cons.