Welcome, Steve! You're definitely in the right place!
"Hard"...I use it for any bait that I want tough/stiff...it's more-so a Saltwater plastic. There are Softeners that can be added that will soften it, and Scents and Colorants will also soften plastic.
"Regular"...I just ordered some Regular, and don't have any prior experience with it and how soft it is, before or after Colorants/Scents, so I'm not sure how much softer it is than Hard, but Regular seems to be the Staple/Standard with most baits.
"Soft"...I don't have any experience with it either, but I've had some "Super Soft" store-bought baits in the past, and they were QUITE soft. If I was trying to get tons of action into a bait, and toughness was NOT a factor, I'd give it a shot.
There will be a few of the Pro's chime in, that have the experience...I just wanted to throw my $0.02 in. More Knowledge/Experience is on it's way, Steve!!!