Here is the official info from Silhouette
As always, the most important information is between the lines. To sum up, the Curio lets you load and cut/draw, etc. (but not necessarily cut through) thicker materials, while the Cameo lets you load and cut/draw, etc. much larger materials. The Curio is configured for ease of embossing, etching and stippling (but with the same downforce as Cameo is still underpowered for these tasks), while the Cameo is configured for ease of paper and vinyl cutting. The thing you really want to look at if you are considering a Curio is samples by everyday crafters and compare those to what your Cameo can already do, or what other Cameo owners are doing.
There are some great Curio samples at this German blog and some great Cameo samples at Amy Chomas’s blog to get you started. My initial opinion is that the Curio does a tad better embossing on vellum and card stock, but about the same on metal foil. This makes sense because with the same pressure you will still get a slightly better result with the thicker cushioning the Curio’s extra clearance allows its embossing mat to have. Hopefully we will get some side by side comparison samples soon.
Here’s my comparison of the features, taking into account 3rd party accessories people have already been using to emboss, etc with the Cameo.
I will also add that if your main interest is embossing or engraving, you will likely be better served by a higher pressure machine like the Pazzles, Zing etc. if you can afford it and are not already heavily equipped with Silhouette library images. So be sure to check out samples from those machines, too, if this is your main interest.