You're buying a high quality sofa. So cushion filling is simply a matter of what feels most comfortable to you.

I'd stay understand what each entails, how it will hold up and what it feels like; cost difference. Then if you haven't already, go sit on sofas with each of those pillow types at the showroom and see which one you like best. Take your time with that. Take a friend/spouse/partner.

I sat on dozens of sofas before I found one that felt right to me.

As for fabric vs leather, I don't see a difference. You've buying a quality sofa. If you get a durable fabric for it you will have years of service.

thanks for your response. you're right that money doesn't matter -- they are all very close in price, and nothing compared to price of the overall sofa!

i guess i'm worried about long-term issues like sagging with down and so on. also, the local retailer where i tried the cushions didn't tell me about "fiber down" but apparently it's an option so i was trying to assess what that was.

Standard Fiber down cushions are are blend of feathers and down with conjugated fiber. Usually 50-50 meaning that half of the fill is conjugated fiber and the other half is feathers or feathers with some down mixed in usually no more than 25% down . This mixture provides more filling power and makes a firmer cushion than would be if all feather/down blend. I would stay away from any cushions that didnt have at least 10% down and goose feathers is better than duck feathers.

When you sit on a down, or mostly down, sofa cushion it leaves the imprint of your body when you get up. It is the nature of the material. It is not a wear issue at all. Over time, when the cushion gets worn down it can be refilled by the upholsterer.

The highest end sofas have those types of cushions so they have that look. It's not wear.

The long term issue I would be worried about is fabric durability, not construction issues. That's why you're buying a high quality sofa.

And these days, a good sofa should wear for at least 20 years. It should not need replacing (even if the kids do jump on it occasionally). I've had a really good love seat now for 33 years. It's in its 3rd and last reupholstery and the cushion was originally and always has been down. It's been refilled for the 3rd time, too.

It will surely outlive me.

With down you really have to sit on it. We rarely sell down, and the few we have, well, I didn't care for it.