Have only done new carpet 2 or 3 times, so this is layman info:

In the store, when you start picking out the colors and brand, flip the sample over and examine the underside. Cheap carpet will have the plugs in a ~1cm square grid. More expensive carpet will have a much tighter grid, some to the point that the back is almost smooth and comfy.

No matter how nice the topside is when you buy it, if it's the more spaces out grid it will wear faster because the fibers are farther apart and not mutually supporting each other. Thick expensive carpet will take years longer to look "worn" than sparser carpet. There are several medium levels between, so you can decide which level fits your traffic expectations vs cost vs how annoyed you'll be over time.

Other thing, The pad makes as much difference in how "nice" the floor feels as the height of the carpet pile does. A 3-pound, 1/4" pad (lowest) will feel like you've put a rug down on a hard floor. A 10-pound, 1/2" pad is probably overkill and will feel like you're walking on a pillow. 6 or 8 pound, 3/8"-7/16" is the normal range for a "nice" above builder grade install

Edit: I stopped replacing carpet in my kids' rooms because it's a pain in the ass. I now replace carpet with a hard floor and put room-size carpet remnant rugs down, they're usually $150-ish at Lowe's/HD/etc and last 4-5 years before they look like crap