EDIT: is there a way to donate to blockchain.info? I couldn't find anything on the website.
In the past we have been afraid of updating the app due to Apple's anti bitcoin policy. It seems like this policy may have changed now though.
Any donations appreciated 16gKb8fYGdvrmuMmir6mQG5MN6nUroriF3
This provides a black and white simple to understand choice for users.
Really the application should be able to determine the appropriate transaction fee better than the user (don't forget many users will not even know what KB means). There is always the custom send form if you want to choose the fee manually.
Makes sense. May be something along the lines of:
Allow Free = if (Target Number of Blocks To Confirm By * Average Block Size 24 Hours) > (Total Unconfirmed Tx Size With Higher Priority Than My Transaction)