I've used both. For a time I used the add-in alone then I got hold of MLO and used it mostly for a someday list - it has a feature where you can pop up a window and capture a passing thought very easily then return to your work.

I got off the add-in and switched to Clear Context Inbox Manager - it does a lot of the same things and just "felt better" to me from a usage standpoint.

I would try the trial of MLO and see how much you find yourself using it.

Clear Context is worth looking at too - it does a lot of the same things as the add-in.