I do use both. Due is more simple, but has lots of features, where as 2Do has LOTS of features but is more of a tasks-type app that utilizes a calendar / planner-type interface.
Due would be if you have little to moderate amount of tasks, where as 2Do would be for moderate to high amounts of tasks.
Due also has a timers tab where you can set a timer, say for something you're cooking, etc.
I prefer Due, but 2Do is a fantastic app and has LOTS of features. If you put in lots of reminders and tasks, 2Do would be for you. As I said, Due has lots of alert / reminder features.