Iceland is full of geothermal heated pools or "lagoons." In fact, you can find them anywhere... not just in these 2 main attractions. As it is out of this world in either case, I would go for both. It is a once in a lifetime experience for sure. If you're fluctuating back and forth, here is my impression of the 2 lagoons.
Let me know in the comments if you've had any experiences at these lagoons, or any other lagoons!
The Blue Lagoon
Price: Standard Entry-$60 Located near Keflavik
The Blue Lagoon is one of the most visited tourist spots in Iceland, and it gets very crowded. The water is vibrantly, unnaturally blue, which is a sharp contrast to the black lava rock that surrounds the area. The lagoon has a plastic floor, and the water is mildly warm, not hot. The steam can make it seem more secluded than it actually is, as you can't see far distances. It's a very large area in general, to accommodate for the amount of visitors. Everything feels very luxurious, there are swim up poolside bars, and you can even get silica mud masks to put on! Another thing to keep in mind is that the area does smell very sulfuric, and the lagoon has a high salt content... don't get it in your eyes. It is also not the best for your hair either.
Price: Standard Entry- $25 Located in the Golden Circle
The Secret Lagoon is smaller than the Blue Lagoon, but also has more of a charming rustic vibe. It provides a scenic view of moss and trees. You can watch from the pool as smoke rises from the surrounding streams, or you can get out and follow a pathway around the streams. The water is much warmer than the Blue Lagoon. The floor of the lagoon is an artificial pebble material, so you won't step on any jagged rocks, but it will still feel "natural." No need to worry about being in an actual lake, though it may look like one. Unfortunately, the water isn't a crazy cool shade of blue... it just looks like an very clean lake. The lagoon is very small, the size of an average pool, but it also doesn't have as many tourists. It is located in The Golden Circle, making it accessible to those driving that tour, which I 10/10 would recommend. There's even a little stone building that makes for a really cool backdrop to pictures.