People use case study and testimonial interchangeably. They're related, but they do different jobs. The simplest way to put it: a case study is a longer form of a testimonial.
A testimonial is straight to the point. Here's a customer, here's why they're happy, here's the problem you solved. It's short, it's punchy, and it's built to be believed quickly. Someone watches it and thinks, okay, real people are getting value out of this.
A case study goes deeper. It's more of a brand piece about that customer. How they came to you, how you found the right solution for them, what changed once they had it. It tells a story instead of making a point, and it runs longer because the story is the value. A testimonial might be a minute. A case study earns two or three, and it's produced with more care because someone watching it is paying real attention.
The useful way to tell them apart is to ask where they sit in the funnel. A testimonial lives higher up. It's not quite awareness, but it's close, the kind of thing that catches someone who's just starting to take you seriously and gives them a quick reason to keep paying attention. A case study lives lower down. By the time someone's willing to watch a full story about your company and one of your customers, they're interested. They're close to buying. They're looking for the last bit of trust before they commit, and a case study is built to give them exactly that.
So which one closes deals? They work together, but if you're asking which sits closest to the actual close, it's the case study. That's the one someone watches when they're nearly there and want to see the whole arc before they say yes. The testimonial does the volume work earlier, earning attention and stacking up quick proof. The case study does the convincing at the end.
That's also why you don't really choose between them. Testimonials are cheaper to produce, and you want a lot of them, scattered across your site and your social feeds doing the early lifting. Case studies are a bigger investment, and you want a few strong ones aimed at the people who are nearly ready. Think of it as coverage at two depths. One gets someone interested. The other gets them over the line.
A quick example of the split. A software company might run a dozen short testimonials, customers on webcams saying the product saved them a few hours a week, and pin those across the site and LinkedIn. Then they produce two case studies, full stories about a flagship customer who overhauled a process and what it did to their numbers. The testimonials get a stranger to take the company seriously. The case study is what the buyer forwards to their boss to justify the purchase.
If you're trying to figure out which you need more of right now, look at where deals are stalling. If people aren't paying attention in the first place, you need more testimonials. If they're interested but not committing, you need a case study. We can help you sort out which.