Mattress Forum

I am new to this forum but hoping I can get some advice.  Researching mattresses is overwhelming!

We currently have a queen Dormia mattress, which was replaced approximately 4 years ago because the sides sag.  The current one has developed the same problem, so we will probably make it a guest bed and buy something new.  My husband has been considering a few types and has narrowed it down to the heavenly bed or a shifman mattress.  I would appreciate any advice on the pros/cons of these two brands.

Also, I now found out that the shifman mattresses are different at Stickley and Bloomingdales.  Does anyone know the difference?  Am I better at one store than another or should I just consider which mattress I like the best.

Finally, we are not certain whether we will go king or queen.  My husband and I are not large and we no longer have small children in the house, so we have not ruled out staying in a queen.  We also feel it will be easier to fit in another bedroom if we ever move.  Everyone we speak to keeps saying definitely go king.  I know this a personal comfort decision, but am curious what people tend to buy these days and why.  If we do a king, we are considering two twin extra long mattresses to reduce the movement between the sides.

Any advice is definitely welcome.

Thanks.