Do I want progressive or linear springs in the rear?

Well, not in my rear... but you know what I mean.

The rear is sagging and I want to restore ride height. I'm not going to autocross but I do want to be able to haul heavy things from time to time. I'm leaning toward the Moog progressives, but I figure there may be some wisdom I can seek on the issue.

progressive,. most rear springs are,.

If you do not have the load leveler working, install air shocks, and a fill valve. When you need the extra lift, fill them.

If you feel you need more spring with increased loads, put coilover shocks on the back, and set the springs so that they only are used when a lot of weight is added to the car. The progressive springs will accomplish this to some extent, by compressing easily to a point, then becoming harder to compress with more load.

As far as springs, the progressives will give you a softer ride over general road variations, and the higher rate will keep it from bottoming out the suspension over large deviations. A linear spring will be a little more bouncy over minor variations.

There are so many spring threads in this forum it'll make your head spin
I spent the winter reading them, like a gardener with seed catalogs. Even had to take notes to keep everything straight like an undergrad research project.
Lots of information, options and varied opinions.
Add the variables of spring/shock combinations, and how it will effect - or be effected by - front end suspension choices, and the possibilities are endless.
I could tell you what I decided, but since I haven't installed them yet I have no actual experience to share.