Photographers: Can you tell me what difference between a 50mm 1.8 and 50mm 2.5 lens for Canon?

So I understand the aperture is set to either 1.8 or 2.5. But what are the benefits of the two, what are the pros and cons? I use a Canon 60D. I hate the stock lens the 18mm-135mm because it seems to take forever when trying to capture a close up photograph. I’m assuming since the 50mm is set, it won’t have to “think” as much and will just snap a “detail” style shot with no problems. Thanks so much, Happy Thanksgiving!