The Peñas Blancas crossing, Nicaragua to Costa Rica · Nicaragua · North America
Crossed overland from one into the other. The contrast was sharper than I expected.

Tested by Paulius Pikelis · 140 countries
Nicaragua and Costa Rica share a border, a climate, and a coastline, and almost nothing else. I did them back to back on the same trip, crossing overland from one to the other, and the contrast was sharper than I expected. If you are choosing between them, it really comes down to what kind of traveller you are.
Costa Rica is the polished one. It is set up for visitors, easy to move around, safe-feeling, and rich in low-effort wildlife and activities. It is also expensive, often startlingly so for what you get, and it can feel processed: a country packaged and priced for a steady flow of tourists, where several of the headline experiences turn out to be ordinary once you have paid for them. You will have a smooth, comfortable trip. You may also feel, as I sometimes did, that you are paying premium prices for a standard product.
Nicaragua is the raw one. It is cheaper by a wide margin, rougher around the edges, less polished, and far more alive. Colonial cities with music spilling into the streets, active volcanoes you can drive up to, adventure that has not been sanded smooth. It asks a little more of you and gives more back. It was my favourite of the two by some distance, precisely because it has not been tidied up for visitors yet.
So: if you want easy, safe, and comfortable, and you do not mind paying for it, go to Costa Rica. If you want character, value, and a bit of grit, go to Nicaragua. And if you can, do what I did and take both, because crossing from the polished country straight into the raw one tells you more about each than either does alone.