Ground cinnamon is less expensive then cinnamon sticks. I've read that it is also more flavorful. However, cinnamon sticks last a lot longer. Also, ground cinnamon is likely to have mystery ingredients. Will I (a cinnamon nut) be disappointed if I stock up on cinnamon sticks instead of ground cinnamon?
Different applications, sticks are mainly used when you wish to infuse a cinnamon flavour in a dish, where the spices are removed at the end, a biryani is a good example. Ground cinnamon is used when the spice is to be left in, or be present throughout a substance, like in cakes for instance.
Ground cinnamon will leave a stronger flavour partly because it is left in and possibly due to the flavour extracting easier due to higher surface area and broken structure.
Cinnamon is notoriously hard to grind smoothly, so people tend to buy both for their respective uses.
Orbling brings up an excellent point about the difficulty of grinding cinnamon to a smooth powder. There are two main different types of cinnamon sticks: Ceylon and cassia. Cassia sticks are thicker and more stiff. Ceylon or "true" cinnamon resembles more of a rolled up parchment and has more delicate, sweet taste to it.
One reason to purchase sticks would be to roast cinnamon (easy to do in a cast iron pan)then grind it. Ceylon is easier to grind because it's a thinner stick.
I purchase both because each serves different purposes.
I highly doubt that ground cinnamon is "likely to have mystery ingredients". Ground spices are just the whole spices, ground up.
In fact there's nothing particularly unique about cinnamon in this respect. The reason to buy ground spices is the same reason to buy ground meat: Because you don't have a reliable spice grinder, or just for general convenience.
Cinnamon sticks don't last forever, though. I would still try to use them up within a year or two.
There is a distinct difference between Ceylon (true cinnamon) and Cassia cinnamon which is what is primarily sold in North America. Cassia cinnamon has large amounts of coumarin.
This and other things are mentioned in this blog post.
Also, Ceylon will grind up quite nicely in a spice grinder, the Cassia does not.