Fusion 4 vs pocket sprung and latex topped mattress

Hi there,
I am very keen on getting a full latex mattress as my current mattress dipping after only a couple years. The best matress I've ever had was one with a couple inch latex top and pocket springs underneath, I am considering going all in on latex but I don't fully understand how it is going to feel. I weigh 14.5 stones so I like the idea of the firm to medium feeling but I suppose I just wonder how it will feel in comparison to a pocket sprung, I know this is a really difficult question but I suppose ultimately I am keen to just get more information before I make the plunge. I've never brought a mattress before trying it so I am fairly new to this but it sounds like a good plan because you do a 30 day return period.
When it comes to sagging mattresses, how does latex fair? Also, what does 80kg latex vs 70kg latex actually mean.
Sorry for all the questions, hopefully it at least makes a little sense :-)
Cheers
Tim