There are indeed two types as noted. My neighbor, who is a Portland electrician, noted that the "divorced" type is often the most flexible type to use as doing so allows you to use any relevant product from any manufacturer. Additionally, as noted by Mr. Lent, using a "plug in" type which is not the same brand as the panel will not satisfy most code requirements. So, as I noted, the "divorced" type generally allows the most flexibility provided you have the room next to your panel box, etc.