If you are going to focus solely on PvP then really there is a clear choice here. You want to go rogue.
There is one very simple reason for this, and it has nothing to do with Warlocks being "weak" in PvP. It's simply the fact of soul shards. Many, many warlock abilities revolve around using Soul Shards as reagents.
It's a general consensus that if a Warlock is to be effective and use all his PvP abilites, he needs to farm Soul Shards for around 30-40 minutes, for 10-15 minutes of actual PvP. Since other PC's do not drop shards this means farming mobs. This is okay for me on a PVE server because I only PvP when I want to, but if you're on a PVP server where you can potentially be jumped at any time, the danger increases exponentially for a Warlock. Many times no Soul Shards = Death.
This is a problem in my opinion, and there is currently a petition to Blizzard posted on the main site with hundreds of signatures for PC's to yield Shards in PvP. Until that happens though, if it does, you're better off playing a rogue if PvP is going to be your main focus.
Edit: I wanted to edit something and note that I think that 30-40 minutes of farming to 15-20 minutes of PvP is a little exaggerated. It might take you 30-40 minutes to farm the shards but unless you're shadowburning every chance possible (which you shouldn't be), you can last at least the same amount of time in actual PvP. I personally think that even this is unacceptable, but my OP stretched it a little.
Edited, Thu Dec 9 17:52:42 2004 by Palison
Edited, Mon Dec 20 05:30:17 2004 by Palison