I've used both. Both worked and carried heavy loads well.
I don't have a "golden butt" to be able to perceive particular differences though.

I'd just find the best deal on an overload spring or variable-rate spring that's spec'd for your car and go with it. I'd prefer variable-rate if you can find it at a price you can accept.

I suspect IPD gets their springs from one (or several) of the existing major spring manufacturers. I don't think it will be significantly different from another overload or variable rate spring.

I'm sure the vast majority of their stuff is sourced from such companies. The value in what IPD does is they find what's useful for Volvos and provide it in an organized way. Considering the amount of hunting we sometimes must to do find special purpose items, that contribution is worthwhile. I believe IPD makes their own sway bars; those are good enough that IPD has earned an excellent reputation for them.

May as well change your shocks at the same time, you need to unbolt them to get the springs in if I recall correctly. BTW, you won't need a spring compressor. Just a jack or two and jack stands or blocks. Occasionally the spring's lower retainer's bolt rusts solid, you might need heat or a Dremel-type grinder to get it free. Usually comes off without trouble. Gotta have a half-inch drive socket set with breaker bar; 3/8 inch will not bust those things loose.
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DAMHIK: Don't Ask Me How I Know - - - Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 500 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).