They all sag over time and heavy use. OME and RTE both publishe the spring rates of their products. If you're comparing an RTE "heavy" spring to an OME "medium" spring with the same loads then yeah, the medium will yield more compression. Also know that the same spring will behave differently depending on which truck it's fitted to - D1, DII, RRC or Defender, because they all weigh different things to start with.
Both RTE's rear application springs are variable rate. Light/medium duty spring is 285-325 lb/in, and the heavy duty are 300-340 lb/in. OME's common rear "medium duty" spring is 220 lb/in, by comparison. OME heavies are 300-340 variable rate. OME has an even higher rate spring that's 360 lb/in.
EE has a great breakdown of spring rates and application data:
http://www.expeditionexchange.com/ome/indexsprings.php
Likewise, RTE's info can be found on their site at
http://rovertym.com/ and clicking on springs.
I myself prefer a much softer rate spring and to carry less stuff. They articulate a bit better, but the big advantage for me is that the lighter rate springs soak up washboard with less bounce. I had RTE heavies on my RRC and they were very stiff. I ended up swaping a bunch of stuff around using a combo of RTE and the genuine 1" lift "police" springs on the front axle.
Dialing in the best spring for you depends on what you're running for bumpers, winch, armor, what loads you carry empty / on-trip, and terrain. All of it will bolt up the same way.
The devil is in the details, as they say.
cheers,
-ike