When do you need a beta reader, or should you find a critique partner? The answer is yes.
Any writer serious about creating the best story they can will probably find they need both a critique partner and beta reader.
What’s the difference? A critique partner is usually a fellow writer, or an editor, who reads a manuscript with the intention of pointing out the story’s strengths and weaknesses. Good critiques often ask questions to help the author think through some of the choices they made. Things like: is this the best POV to tell the story from? Is the story structure solid? An so on. Questions and comments a general reader is unlikely to ask, but the author needs to consider.
A beta reader is a reader. Usually and avid one, and often focused on a group of favorite genres. Most beta readers are not experienced writers (although many experienced writers also beta read). But a beta reader’s feedback is important because it tells a writer how a typical reader might react to the story. Often a beta may point out some confusion about something in the story and the author realizes they haven’t actually written all of the words that were in their head.
So is a critique partner is better than a beta reader? No. Each perform a specific function. Barbara Linn Probst reported on the findings of a study she did which explains how they are different and when each is used.
For genre writers, it’s important to have genre knowledgeable beta readers and critique partners. The reader will tell you if the story is working in its intended genre, and the critique should tell you if the genre story is working.