Most retailers/manufactures offer several cushion options and quality will vary depending on how their made. I will try and answer based upon a previous posting.
"Furniture manufacturers are in business to make money, just like everyone else. Your local furniture store buys from the manufacturer through the manufacturers sales representative. This representative also earns a percentage of all sales in their territory. So it should be common knowledge if you can purchase directly from the manufacturer you can avoid the retail store markup and also the representative take from each sale.
I suggest that to partial38, the features your looking for may not get you the best deal, or fall with in your budget. A high quality frame? What this is, is not a pine plywood frame stapled together. Solid hardwood 4/4, 5/4 is better or hardwood woodplywood thats same thickness, minimum of 12 plys. This frame should be double-doweled and glued.
Now the seat springs is where most everyone makes the mistake that it has to be coil springs that are 8-way hand tied. And thats simply not true. A no-sag sinuous wire spring; 8 gauge spring securely fastened to a hardwood frame using the clips that are required by the spring manufacturer, and that are engineered to the correct length and stretched accordingly to the spring manufacturers requirement will result in a seat suspension that will last a lifetime and have the same ride/push-back as the best 8-way handtied coil springs. The only time that a 8-way handtied coil springs suspension is better is when it is a spring edge with the correct rail heights all the way around the frame. At this time I dont know of very few manufacturers building their 8-way handtied coil springs this way, if so the costs for a leather sofa will be upwards of 5000.00 Its just too labor intensive and takes highly skilled worker to build.
Truly the only type of seat cushion core that will last for 20 years+ is a 100% natural latex. A true 2.0+ pound density high-resilent foam core will normally last 5+ years. But all will loose some shape because of the wrap around the cushion core is of a softer material: polyester fiber or down blend or a thin foam wrap of same density foam which last better that the first 2 , and all manufacturers dont consider this softening of the softer wrap a defect in any way, but just normal wear. Back cushions are usually of a the same: polyester fiber or down blend and will not last or hold its shape after considerable use. But usually thats a fix the owner or upholstery shop can do to replace this filling to be like new again with minimal costs.
If anyone does a search of my name in this forums will realize I have been preaching this for many years. My 38+ years of managing upholstery product design and development for 20+ manufacturers and many years working in re-upholsery. I have always said that your local re-upholstery shop that has been in business for 20+ years with the same owner is the best resource for understanding what it takes to makes a quality sofa. Because they have to repair or junk the ones that arent worth re-upholstering."

I'm about to post a very negative diatribe against Hickory Park Furniture in NC because of a sofa I bought through them. I specifically told them I wanted a firm and resilient cushion. They said that would be the Spring Down Hamilton cushion. If that cushion is firm and reslient, I'd hate to see their soft and non-resilient cushions. Even CR Laine, the sofa manufacturer, has told me that for someone wanting a firm cushion, the spring down Hamilton is NOT the right one.

So, I'm trying to unload this hideous sofa and have ordered a new one from Hickory Chair (Ouch!). But I did my homework on cushions first and read that the absolute best cushion for firmness and resiliency is a cushion made with Bayer Company's (the aspirin company!) 'Ultracel Foam'. Hickory Chair (great customer service, btw) has agreed to use this foam for my new sofa. I can't speak to it, though, as I don't have it yet, but according to a cushion expert, it is the way to go. He calls it the "Rolls Royce of sofa cushions".

http://www.letstalkfurniture.com/interview-everything-you-wanted-to-know-about-furniture-cushions/

Its great that your doing your homework before purchasing. However, you should check the ULD or IFD of the foam. ULD determines just how firm or soft a seat cushion will be. Anything in the range of 22 to 25 ULD will be a medium soft seat. A ULD of 26 to 28 will be a medium and a ULD of 30 to 32 will be a medium firm seat. I dont recommend less than 22 or more than 34.
Yes 2.5 density ultra cell is better than 1.8 density ultra cell, but an ultra cell foam not the "Rolls Royce of sofa cushions"
A 100% natural latex is the best you can buy.

Unbiased info about foams can be seen here:
http://www.pfa.org/intouch/

Bayer info here:
http://www.ultracel.com/

Here is a link that might be useful: 100% natural latex sofa cushions