Depends a little on your build, but the big three are the 2 you listed and poison. In PvP, disease is really nice as well because it limits healing for the person you are fighting. If you are fighting undead or daedra (most of IC content), you cant beat the new prismatic weapon enchants or whatever they are called. I run with one on my main bar most of the time.
In terms of Fire vs. Posion, there are two things in play. First, as a general rule, Fire hits harder up front, and Poison does more damage over time. What is probably more important is where you put your CP. If you are stacking into elemental damage, than Fire makes sense. If you are stacking into thaumaturge, then poison will likely be better.
On my DK, I carry 3 staves. I carry a Fire and Daedric enchant in Nirn for my front bar and switch between what I am fighting. I always run a precise staff with poison on my back bar for AoE and the extra DoT. Hope that helps a little.