Nowadays it is very common for people to use email subscription services like that of BookBub, Book Gorilla, Book Daily, or Book Bear in order for readers to notice of new writers when put on sale. The service offered by each is different from that of other, with varying prices, and audiences. I’d like to compare Book Gorilla with BookBub and The Fussy Librarian.
a. Well to start with, BookBub has a huge base of audience compared to others and the service quality offered is also high. They also have special offers which will increase your reader base dramatically.
b. BookGorilla, a Kindle Nation Daily’s part, though having a huge base of readers, advertise with very long emails which people tend to delete even without opening from the inbox. Still, they are a lot cheaper than that of BookBub.
c. The Fussy Librarian is not like the other two, but it is a free service. So, even when you do not reach many audiences, still you can give it a shot. But another setback of using this is that they have highly customizable service which enables customers to see only books which they want to see, making it more unlikely for your book to be seen.
My recommendation is that you should go for BookBub is where people would recognize you, this way you get to build a name for you. Then you can go for something free like The Fussy Librarian and advertise since you already have a good name.