bc girl I find Books on Board to be very good, and they hold a great volume of titles, including titles that Kobo doesn't show for me.
Here's their latest email:
'Check out tomorrows eBook releases at BooksOnBoard and with promo code janbook you net 1/3 OFF the publishers list price on all discount eligible eBooks and all of our Audio Books; after Promo Code + Rewards Dollars back.
If you haven't seen the new BooksOnBoard App, check it out here: BoB Reader App.'
Re preference of ebooks? I'm not sure that I've noticed a difference in quality between any of the providers. I think that sometimes there may be odd characters or some such in some of the ebooks, but, luckily, that doesn't seem to happen too much to me.