Yes, they are both forms of the same squash and are both delicious. Butternut is the most common in the US, and buttercup squash is more common in Europe. If you're shopping for a butternut squash, you're probably looking for a heavy-duty cooking type, but if you're shopping for an "average" buttercup squash, you can get away with a little less.
Buttercup squash has a light orange color with a yellowish interior. It's also much smaller than butternut squash, usually around 10-12 inches long. Butternut squash is usually larger, around 15-16 inches long, and it has a white interior with a very dark orange exterior. You can use both types of squash interchangeably in recipes. But for the best flavor, I recommend using butternut squash. Buttercup squash is great for stuffing, but its texture isn't as nice.