I've used a 5th gen nano for years, and use it exclusively for my running (with the Nike+ Bluetooth receiver on iPod and motion sensor on shoe). I don't use a case, and hold it in my hand when running. I've taken a few falls, and dropped my nano several times. So it has some scratches. Still works 100%. From my experience, it's sturdy. I'm sure there are some owners with broken click-wheel. When tens of millions of anything are sold, EVERY potential type of problem will occur for the overall customer base. That does not mean any particular problem is likely to occur to any individual customer. Since you don't touch the screen to use a 5th gen nano, there is no issue with fingerprints. As a long-time iPod user, I like the click-wheel interface.
The 5th gen nano is becoming an "old" iPod at this point. Old things tend to start breaking down. If the iPod was heavily used, its battery is likely to be worn out (or getting there). If the iPod was not used very much, and sat in a drawer, that can be even worse for a lithium battery. The 5th gen nano has a battery that is soldered to the logic board, so it's not easy to replace, unless you have good soldering skills and enjoy taking apart miniature electronics.
Unique feature of 5th gen iPod nano: It has a video camera (for taking videos not photos), along with a built-in microphone and tiny speaker.
The 7th gen nano is the current model. It's brand new with a one-year warranty. Users who have never used a click-wheel iPod may find it more usable. It has a touchscreen interface; you'll deal with fingerprints. It uses the Lightning connector, which is the current standard for Apple device docking. No Nike+ accessories needed to track running.
Unique feature of 7th gen iPod nano: It has built-in Bluetooth.