I do mostly small baits and basically use nothing but the soft. I know that some baits might have a better action if a medium or firm plastic is used and some of the stiffer plastics have more resiliency in cold water applications. You don't indicate what fish will be sought and species/baits will make a difference as to what plastic is best suited. If you are talking crappie sized baits without a tail like a twister or K grub, soft will probably do you best. If you are shooting twister-tailed baits I'd go with medium just for durability and less time in the box when time matters and because I think medium gives those broader tails more action. I'm in Minnesota and took a couple 2.5" and 3" K grubs shot in soft down to the river behind me and played with them to see how they looked in the water...how the action was. The water temp was 35 degrees and the tails were awesome in action.
