My reason for replacing my air suspension was financial and I was looking for a trouble free operation. Simply wanted to forget about the air bags leaks and the cost associated with replace them + the issues that you are experiencing. Since changing it to Old Man Emu springs and shocks many years ago, I haven't experienced any issues at all. I did the job myself and it was fairly easy. After completing the job, I had better clearance and was able to install bigger/more aggressive tires.

This was for my RRC. I do realize that you own a Disco, but wanted to share my experience, from a RRC owner perspective. I hope this is helpful.

 

I would also want to compare the cost of each. And possibly even a more durable air suspension. Possibly upgrade the plumbing to a more durable material/construction like the aircraft style pneumatics.

All depends on your degree of disposable income compared to your overall desires.

Adjustability may be nice while a really good heavy duty coil spring/shock combo may better suit your specific needs.

 

I like the air SLS on my D2 because the ride is nice and after overloading the back of it with wood/building supplies and a trailer it comes back up to normal ride height. And it still rides comfy with a load in it!

Add to this I can raise the rear by a good 3 inches when off-road with the push of a button!

I just had the same issue the OP had with saging rear when parked. Used soapy water in a spray bottle and discovered a pinhole in the left spring. $230 for BOTH rear bags WITH a lifetime wty, now the truck rides like new - problem solved.

No trip to Land Rover to reset/disable the "system."
Easy repair in your own garage.

 

You can also add up to a 3" lift along with the SLS and have the best of both worlds.