Sagging over time, means that the coil lost some of its spring coefficient -- some of its 'springiness'.
The better coils, which are made from better spring steels, do not sag over time.
If the harder part of the MC coil was holding the weight, it would have meant that the softer half of the coil was completely compressed, and the ride - on the harder half of the coil - would have been hard.
It doesn't work this way, and the fact is that MC dual-rate coils give relatively soft ride.

The soft half of the dual-rate coil is whats' holding the Jeep at the specified lift.
Only after this half has compressed, the harder half starts compressing, and softens the downwards movement of the Jeeps' body.

 

IMO, the lighter spring rate at the top of the MC coils due nothing for the ride,they are just used to keep the spring from falling out of the bucket at full droop when running longer shocks. They are at full compression at ride height, leaving all the work to the heavier rate portion of the spring which to me seems like a linear spring with a built in spring tender,same as a coilover would use on a triple rate set up.

 

I checked, and you're right. My mistake.

As for coil sag over time, this stands. Better coils do not change this way over time.

 

So how do you know what coils are considered "Better?" Someone sent me a PM telling me to buy the RE 4.5" coils. Anyone have experience with those?

 

RE will be cheap, made in China with whatever steel they have at the time.

 

Well, peoples' opinion vary, but when you see that the vast majority give positive (or negative) feedback, that's usually a good lead.

OME, Full Traction and Metal Cloak are widely considered as relatively soft coils.
I have 4" lift Full Traction coils. With re-valved Fox shocks, the ride is softer than stock suspension.
I didn't have a chance to drive a Jeep with dual-rate MC coils, but from what I read, there's a consensus that they are the best of the three. No wonder, as dual-rate or progressive coils would give better ride than regular coils.

In the pair of coil & shock, it is the shock that has much more influence on ride softness or hardness than the coil does.
Therefore, if you install MC coils but the shocks are not properly valved for them, your suspension might be too hard (or too soft). I suggest consulting with MC about the shocks, or getting driver-adjustable shocks.
I like the Fox shocks.
King may be marginally better and cost more. You probably won't be able to destroy a Fox shock.

 

I would not make the blanket statement that Old Man Emu coils are soft.

OME makes several different coil rates for the JK, their no load coil will have the lowest coil rate and be the "softest" ride. The HD+40 on the other hand is made for a very heavy vehicle and if installed on a light Jeep will feel rough and will sit up much higher then the advertised height.
IF you are looking for ride quality you need to take into account the weight of the Jeep anyhow you drive so you can pick the proper coils for your desired height and like said above custom valve the shocks to fit the coil rates and what you are doing with the Jeep as there will be different valving for street, rock crawling, and desert racing. A shock with adjustable compression or bypasses will allow you to fine tone the ride to your butt but can be expensive. For the cheaper solution pick a company that offers tuned shocks to fit their coils instead of trying to mix and match with the generic throw a way shocks that are out there like the 5100 or sealed mono tube shocks.

Remember there are a lot of factors that will go into ride quality and what may be good for one persons set up may not be good for you.

 

Obviously, I referred to the softer OME coils. Anyone who'd search for their shocks, would see the selection anyway.

If valved for the offroad type that one usually does, the shocks won't be ideal for occasional offroading of another type. In addition, personal preferences - or possibilities - in offroading, may change over time.
That's why I'd go for driver adjustable shocks, rather than make a compromise between road and offroad.