New DIY Build: 28ILD HR Foam vs 30ILD HD Foam Support/Foundation Layer

Hello friends!

I’m back, and this time I’m building a bed for two. My current build is great, but we’d like a king size so I dug back into the DIY route. The build I have currently only used a 3" support layer just because I had to remove a piece of the initial build. I was hoping to jump up to 4" for the lowest layer just for a touch more firmness.

I’m debating between a 4" support/foundation layer of either:
28ILD HR Foam
30 ILD HD Foam

There is a pretty sizeable price difference between the two, so I’m just wondering if it’s worth saving or spending the money since I already have a reference point that I am happy with. In other words: “Will 30ILD HD be that much of a difference?”

My current build is:
2" memory foam
2" 20ild Dunlop
2" 20ild HR Foam
3" 28ild HR Foam

Thanks so much!