My POS COS has a cooking chamber that measures 20"x~31" and though I've filled it pretty well a few times I've never wanted for more capacity. So I'm not looking for anything larger than that. The offset's I've been considering are those in the $1k - $1.8k range from the likes of Lang, Gator, Yoder & Horizon. There is a guy here in CA making a RF offset but I've yet to make it out to his shop (2.5hrs away, http://katbbqsmokers.com/
he makes a 20" RF cooker that isn't on his site priced around $1.5k) to take a look at his work nor have I found any reviews.
I don't know enough about the pellet cookers yet to identify a brand that I'm officially considering. Nor have I seen any of them in person.