I do both.
There are many authors that I follow and will read just about anything they write. But, my favorite writers often don't write enough to keep up with my demand. So I hunt through the shelves looking for something that looks interesting. We're lucky here in the Minneapolis area to have an excellent SF & mystery/crime specialty book store Uncle Hugo's & Uncle Edgar's Bookstores - Science Fiction, Fantasy, and Mystery Books
. The staff will be happy to discuss preferences, and is great with quality recommendations.