The Acekard is the best card on the market and It is the only one that has an open source firmware.
The original R4 is a good and and widespread card, and because that, the market has been invaded by millions of clones.
Both cards are good for NDS homebrewing, so: 65 or 10 bucks? I would go for 10

http://en.wikipedia.org/wiki/Nintend...torage_devices