So for a malawi cichlid tank.. which one would be better? limestone or dead corals?

In your case as you have hard water with a high pH to start with you don't need to worry about the decor being a buffer, so you can use whatever rocks you like.

Personally I don't like to see coral in a freshwater tank because it's not natural and bugs me aesthetically so I'd choose limestone over that if I needed a buffer, but otherwise I always find slate or smooth boulders more attractive to look at.

That makes sense, but with my experience, algae grow really fast on dead coral, which is a great supply of food for malawi cichlids.

I don't know which kind of decor will bring my tank closest to malawi lake natural environment.

I believe most of the rock in the lake is granite and sandstone

I will try to seek them out! I've never done any aquascaping with just rocks :P