I'm sure someone can give you a techy detailed breakdown about write speeds etc (probably be covered over on Rob Galbraith etc) however as a mark iv user, I use CF & SD cards and have never had any problems. I usually have camera set to automatically switch cards when one is full, that way you can choose when you stop to change cards rather than being forced to stop.
Given SD is significantly cheaper, go for a decent one, it'll still be cheaper than CF....