My library has both and you are right that OD has more popular and recent material. The only things I have checked out from NetLibrary are classics (read online with their reader) that I probably could have found free elsewhere and audio books. Their audiobook selection (at my library) is pretty inadequate and hard to use as an entire book will be on one file. My cheap mp3 player works better when the book is broken up into files for each chapter. My previous cheap mp3 player couldn't even hold an entire book due to file size constraints. The final reason I rarely use NetLibrary is that their search engine is pretty useless and unfriendly.
I had been using NetLibrary to read one author not previously in my library's OD selection but recently they have been added to OD and hopefully I'll not longer need NetLibrary. I do not know if the reason for the differences in selection are due to my library's purchasing habits or just a difference in what is available at NetLibrary. I have recently moved to another region with a different library system and they only have NetLibrary. Their selection is also not very good (nearly the same as my previous library's). I will continue to use my previous library's subscription to OD as long as possible.