Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
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.
 
What is a nicely laid out, simple to-do app with alarm notifications/reminders?

I am in your same boat, but I am holding out to see what the stock option is...I hate having duplicate apps and since the stock one will probably be irremovable I'll just wait, but 2Do is the one I have been looking at.
 
I recommend the soon to be, iOS 5's Reminders app. I'll be using it for quick Reminders, such as "find out about X when you go to Y" or "pick up the package when at work".

But i currently use 2Do, and it's a fantastic app. I tried so many todo apps, and 2Do is definitely the best.
Things is overrated, and overpriced, for simplicity. If you want simple tasks then use the soon to be, Reminders.
 
I have tried a lot. For me, apps like 2Do are great, but for me, the quickest reminder app which stuck with me is called "TellMeLater". It doesn't have many bells and whistles, but it's quick and reliable.

Second honor goes to "NotifyMe" just because you can signup for a free account and you get cloud notifications to email address on-top of local notifications, if needed.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.