Hi a35patel,
I always go by the materials not by the manufacturer to determine the quality and value of a mattress.
They are what is called an ESOP (Employee Stock Ownership Plan) and a forum search on “Southerland ESOP” (you can just click this) will bring up more comments about them. As in all cases though the quality or value of their mattresses (or any mattress) depends on what is inside it and of course on the price it is selling for at the particular retailer you are dealing with.
2.5 lb polyfoam is a high quality foam and 2.5 lb memory foam is a very low quality foam so it would depend on the type of foam you were talking about.
So to know the relative quality and value of any of their mattresses you would need to know the specific details of every layer and component in the mattress … layer by layer. This is the job of the retailer who is selling it to find out and let you know (customers shouldn’t have to track down the specs of a mattress). this means the density and thickness of any memory foam or polyfoam layer, the type and blend of any latex, and the type of cover and quilting.
If you are able to find out the layers of the specific mattress you are looking at and list them here along with the mattress only price and size you are looking at I’d be happy to let you know what I think of it.
Post #4 here includes the better options in the Phoenix area which has some of the best quality and value available in the country including three of the members of this site. From a mattress perspective … you are very fortunate to live there 
Phoenix