The "stiffness" of the dome is much more even, meaning that the force required to depress it is pretty much equal from start to finish, which eliminate the god awful "dome break" that cheap rubber dome keyboards have in order for you to actuate the "switch". This also provides a smooth travel that mechanical switches tend to offer.
That alone makes Topre a lot better already then regular rubber domes, but having the option for high end caps in your case (and stock thick PBT caps on the HHKB/Realforce for instance) makes the typing experience even better while also providing a more solid feel as well as longer lasting caps.
The build quality is also often a lot better in Topre keyboards. The Realforce, Leopold, Novatouch, HHKB are all pretty solid keyboards.
Those are the reasons why I prefer Topre anyway. Others may have better explanations of how Topre works exactly if you're curious about that.