Iritis
Iritis can cause a painful red eye with raised intra-ocular pressure. This picture shows a different complication of the iris in iritis called posterior synechiae where the sticky inflamed iris stick to the lens behind it.
What complications do you think posterior synechiae can lead to?
Conjunctivitis
Conjunctivitis does not usually cause much pain, there is usually some discharge and does not cause iris vessel abnormalities. It also frequently involves both eyes.