My first play took me 4:15. It is short, but at least for the sale price, I thought it was worth it. If you want to try before buying, the original Cave Story is free on PC :-P (Though I'm wondering why Cave Story 3D isn't on the eShop, weird...)
Cave Story's more like a shooter game and has a retro aesthetic. It's longer, tougher, and involves exploration in more varied areas. Steamworld has a more modern graphical style and is almost like 2 games in one, half horizontal platformer and half "vertical" with digging downwards, with a bit...