It really depends on the type of bait you’re pouring and your presentation. Soft= lots of bait movement but does not last long due to teeth or structure. Medium= just that, middle of the road in bait movement and durability. Hard= less movement of the bait but it lasts for several catches or snags.
With that said, some bait types like swimbaits often times call for a medium or hard plastic to maintain their action. Tubes is another I generally pour with a hard plastic for durability. With crappie baits I use medium or soft plastic.
I suggest trying out a few different manufacturers’ plastic sample kits to get a feel for what you want rather than being disappointed in a gallon of money spent. If you do a sample kit, shoot your favorite mold once in clear then try some with color. Do this with all three firmnesses. Let the baits hang for several days to a week and see if the clarity and firmness suit your needs. Make detailed notes with dates in a notebook as you go.