Well, after a few months of using 3M I thought I'd provide an update.
The best thing about this thread was my discovery of the Overdrive search plug-in for calibre. Thank you! I have added many "empty book records" to my calibre library to keep track of book club recommendations, challenge lists, etc. This search makes it really easy to find where I should go to borrow or purchase the book. It's fantastic!
The biggest benefit of 3M is that my library was correct when they said that it will allow them to have books that their patrons want. They've come through on my requests within 2 days on average. Before I would have looked on Overdrive and then bought the book at Amazon if it were missing. Now there's an excellent probability that the 3M library will add it if I request it. My Overdrive consortium has not added any of the books that I've recommended!
My library has been expanding their collection so the selection has improved. I have found many books that are on 3M and not Overdrive so I suppose competition is a good thing....
3M updated their iOS app this week. It fixes margins, page numbering, filtered searches for ebooks versus audiobooks, and some other things that I didn't like. That makes it much more useable! The desktop version is still an outdated, ugly and slow interface.
In summary, I still prefer Overdrive as a library lending service. I'm getting use out of my additional library cards. Having additional Freegal picks per week is great, and I'm really liking OneClick for audiobooks.
Thanks all for your great advice!