I can go either way on this for this project, but I'm curious if using a plist to store some data is going to be more or less efficient than just keeping a plist in the documents folder. The data is about 50 strings/dictionaries.
In both cases the data gets persisted using some file IO so disk access should be similar.
However, the plist seems like a little more work.
NSUserDefaults is a plist (that is why only plist types can be stored in it). So ultimately there isn't going to be much difference in efficiency (whatever you mean by that). Your consideration should rather be where it is appropriate to keep this data. Don't keep it in the Document folder unless it is appropriate for storage in iCloud, says Apple; it will be backed up when the user backs up the device, and will subtract from the user's quota, so you need to be sparing of what you keep there.
In one of my own apps, where I download a bunch of data from an RSS feed and present it to the user, I store the data in the user defaults, because it is part of the app's persistent state the next time it appears. My data isn't a document; it's the app's state. That's my reasoning, and I'd suggest you might reason along similar lines...
In my opinion, plist are much simpler to use than NSuserDefaults. Afterall, a dictionary can save itself as a plist. As for efficiency, they sould be the same as NSUserDefaults stores everything as a plist but provides more services such as comparing which key/values pair have changed compared to a provided set of key/values default pairs.
You may want to consider JSON using JSONKit. Some tests show it's faster than a binary plist, if speed is your primary concern. The API is dead simple because it creates a category on NSDictionary and NSArray. Calling -(NSData *)JSONData on either of those objects returns an NSData object ready to save.
