A great post from Matthew Green comparing different encryption apps for iOS. Someone (I can’t find the source) on App.net sent this over to me and it is a great breakdown from someone who presumably knows his stuff.
Of the apps listed I use Wickr quite a bit. I thought it was pretty secure, but it looks like it is less secure than I thought. I just signed up for Silent Circle and will be testing that out. It’s a much more complicated system, but it looks to be far more secure of a system.