Hi KrazyPanda,
Synthetic rubber is just as “real” as natural rubber. They are both “equally” rubber.
They are both rubber copolymers so if a mattress uses all latex layers inside it and there is no other material in the mattress besides the latex then the mattress would be 100% latex (“latex” includes everything that is part of the foamed rubber formulation).
I would also keep in mind that latex ILD is always in a range (see post #6 here) and there is no exact number for a latex core although a manufacturer may specific a specific number (usually the “target” ILD) for the sake of simplicity. The ILD of a 6" latex core is measured in several places (depending on the manufacturer but it would be a minimum of 9 spots on the original core) and is then averaged and rated by the ILD rating that is closest to the average of that particular core. This means that a latex core that is rated as say 24 ILD may have an average ILD rating that is a little lower or higher than the “official” rating but ILD differences that are less than about 4 ILD wouldn’t be noticeable for most people.
The most important consideration is whether a topper you are considering will be a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) on the mattress you will be using it on. This will depend on the specifics of your mattress, on your body type and sleeping positions and each person’s individual perceptions and preferences, and on the type and blend, the firmness, and the thickness of the topper. The only way to know this for certain will be your own personal experience.
I would also consider the type and blend of the latex which would be a preference and budget choice more than a “better/worse” choice.
I would also consider the return or exchange policy which may become more important if the topper you choose isn’t as good a “match” for you as you hoped for when you actually sleep on it.
The warranty would be my least important consideration because defects in a latex material will tend to show up early in the life of the material (generally in the first year or sooner).
Assuming you are buying from a reliable supplier … I would make overall comparisons between latex toppers based on your preference for different types and blends of latex, on the exchange or return policy (which affects the “risk” of a topper purchase), apples to apples price comparisons (including shipping) for the same type and blend of latex, and of course based on any other criteria that are important to you (such as the number of options that are available or the type of guidance and service a supplier provides for example). If you are confident that a topper will be a good match for you then the return/exchange policy may be less important because return and exchange policies are built into the cost of a topper.
Phoenix