Disclaimer: I am a rank novice. You should ask this question of your instructors and other people who know what they are talking about.
It seems like there are two distinct schools of thought: a cutaway followed by reserve deployment, and just dumping your reserve.
I've heard two arguments in favor of cutting away. The first is that it keeps your emergency procedures consistent -- if you have a malfunction, you clear them all the same way. The second is that if your container is locked up because of pressure problems (which is probable), then the deployment of the reserve is likely to cause things to shift around enough to permit the main pin to dislodge, which would give you a two-out situation.
The arguments in favor of going to straight to the reserve are also pretty good. If you have a total malfunction like a container lock aka PC in tow, you are heading straight for the ground really fast -- your first priority should be getting something over your head post-haste. Another problem is getting the main risers entangled with your deploying reserve, perhaps irreparably.
The SIM mentions both as options, and I've heard skydivers I trust and respect on either side of the debate, so I think this is one of those situations where you have to think about the risks involved, make a decision, and train on it until your response is deep in muscle memory. Personally, I would go for the reserve without cutting away.
The best way to stop this sort of problem is gear maintenance, of course: make sure your closing loop is the right length and that your pilot chute is properly constructed and maintained. Bill Booth made a great post a while back talking about how to check your pilot chute function.
Blue skies! /gdm