I'd stick with 1/2 share for cohorts. Otherwise, you could run the risk of the cohort getting enough xp to leave, once his level is too high for your leadership score. Also, I'd imagine other players wouldn't be happy about giving extra xp to an npc. A good npc, but an npc all the same.
As far as followers, I wouldn't track xp for them at all. Their levels really shouldn't change unless your leadership score does. Some may come and go, depending on the situation, but your leadership score should determine the levels of the rank and file followers.
Also, I'd say cohorts were built for adventuring with a PC, but most followers aren't. They're the ones that run errands for PCs, watch the baggage train, and keep up the keep. If you have a bunch of 1-3 level characters following you, let's say a 9th level PC, into the Dungeon of Doom, you're likely to have a load of dead 1-3 level characters.