Poultices and plasters are very similar in regard to their healing properties. The main difference is that poultices are made most often with fresh herb matter (or dried with a little hot water added) macerated and applied directly to the skin. Plasters are made with either dried herb or powdered herb mixed with a carrier like oatmeal, clay, ground flaxseed or flour to create a paste when mixed with hot water. Sometimes oil or wax is added as well. This is then spread on a cloth and applied to the area and covered with another piece of cloth, which helps to hold the heat in.
Which method an herbalist chooses really depends on what herbs/materials they have on hand, and which herb they would like to use for a condition.
Plasters (because of their heat and oily/waxy nature, and because powdered herbs are quite strong) are typically very well suited topically for the chest/abdomen to really stimulate the internal organs. For chronic chest infections in the fall/winter, musculoskeletal pain, sore muscles, and digestive discomfort, plasters work well as they bring in the heat and help reduce inflammation and pain.
Poultices are wonderfully fast to prepare as you can chew the herb up and apply to cuts, scrapes, wounds and burns to promote healing and reduce infection. Yarrow poultice is wonderful to stop a wound from bleeding and plantain is great to reduce an itch from bug bites.
They would both be effective forms of healing for you but a plaster might be more punchy! Capsicum topically would really help to reduce the pain, which would be effective to add to your St john’s wort poultice or plaster.