I have not had a Casita, but I've had just about every other brand including all three sizes of Scamp. The single thing that Scamp offers that no one else does is a complete custom floorplan in the wood of your choice if you go with the Custom Deluxe models. Two of my Scamps were CD models, the other was a standard. The full wood interior, while slightly heavier, really makes for a more elegant feel to the inside.
Casita offers larger tanks than Scamp which may or may not be significant to you (it wasn't to us, nor are the larger tanks yet in the Bigfoot
a selling point for me).
Scamp offers the 19' fifth wheel, Casita offers the 17' in several floorplans. Scamp has an open treated floor, Casita's is enclosed. None of these features is "better" either way, and each has it limits so it really boils down to what you find important for the way you intend to use it.