Kobo when the ebook takes Vouchercodes40.
Also Books on Board when their discounts include the book I want to purchase, where Kobo may not be showing it for me. It does happen.
Also Amazon if it's cheaper than the rest.
Sony I do check, but more often than not it's dearer than the others, and with a paucity of discount vouchers.
Having said that, I use the Sony Reader for PC as my default download program no matter where I buy the Epub from, and for library books.
Straight from Reader for PC into Calibre. I don't need to open ADE at all.
Did you have a particular query about pricing?