Buy the best sofa you can afford. A good sofa can last 20+ years with a few re-coverings. There are several trade-only lines that I have had very good luck with over the years. There are also some excellent retail lines if you are a do-it-yourselfer.
If you are investing in a new sofa, check the legs to see if they are wood. Many brands use cheap plastic legs which will not withstand heavy wear. “8 way, hand-tied” is also an indication of quality.
Don’t think you have to have down or blend-down. Down backs require constant re-plumping to look good. Sometimes a “tight back” style is the way to go, which has no separate back cushions. Again, a good designer can help you make the right decision.
As for style, you need to decide how you will use the sofa.
In a family room where someone might lie down to watch television, for instance, you would want to avoid a Knowles-style sofa which has very high sides.
Toss-type back cushions will end up on the floor, especially if you have young children. Semi-tight backs aren’t able to be rotated, or even easily re-plumped, since they are attached.
A single bench cushion for the seat is a good look and on-trend, and is more widely available as an option now.
Two seat cushions are not as comfy for someone lying down to watch television, but more comfy if two people are sitting together (upright). All of this is worth a discussion with your design professional.
A good design professional will keep you from making an expensive mistake.