I've been downloading ebooks and audiobooks from both the Seattle and King County library systems for about 3 years now. When I want to find out if a book is available, I generally go directly to overdrive's search site first:
http://search.overdrive.com/ They have a great search function with links to the libraries that carry any given book; you can also search by zip code to find out what (if any) library in your area has ebooks available for download.
Overdrive usually has books available to the libraries before the publication date, and the individual libraries have the option of posting them and allowing you to put a hold on them to have a better chance of getting a copy when the publication date arrives. The library will then email you when a copy is available. Some libraries take advantage of this, some wait until after the publication date to obtain the book. When it comes to upcoming releases that I'm really looking forward to, I tend to start checking the overdrive site a month or two before the publication date, hoping to get on the wait list as early as possible (I mainly do this because it's easier than checking the two library systems separately).
My best guess is that as a general rule the larger your library system is, the higher their budget, and consequently the better the selection. Also, new libraries are being added all the time, and the longer your library has been offering ebooks, the better their selection will be.