Hi Siren,
The main difference between a 2" topper and a 3" topper of the same material is that the thicker topper will be softer and more pressure relieving.
I looked at the links for the two futon shop wool toppers and it seems odd to me that the 3" topper has either the same or less wool in it than the 3" topper (usually thicker toppers have more wool). I would call and ask them if this is correct. There is also more about wool toppers in post #3 here. They will tend to feel a little firmer and less resilient and less contouring than soft foam toppers like latex but they are also more breathable and temperature regulating.
I’m not sure if you have a mattress or not but all of these are meant more as toppers to go on top of a mattress and would feel very firm and likely bottom out if you used them directly on top of your platform bed.
Phoenix