Completely different feel.
Reds and blacks are linear - the just slide smoothly to the bottom and back up.
Topre has greater initial resistance, then is buttery smooth all the way down.
HHKB is 45g Topre, and MX reds are also 45g, but the HHKB feels stiffer due to the different force curve.
MX blacks are much stiffer than reds, many people find them too stiff, but I like them. And I like HHKB. MX reds are too light for me though.