I think Glenn has brought up a good point. The first thing I would do, armed or not, would be to alert the authorities of the incident on bored. Although this probably already has been done, it wouldn’t be bad to do it again, and relay what you see, and have seen as the more information they have, the better they can make decisions. I am not sure where a 911 call from an air phone would be routed, but they would probably know who to call. If you used a cell phone you would probably get the dispatch center for whatever area your over, and they might not know the most expedient way to relay the information (try calling the 800 number for the airline your on).
If it were to come down to shooting, I know aircraft are built very well, I would not be worried about a rapid depressurization, as the chances of small arms fire causing this is very small. My primary concern would be with neutralizing the hijackers before they are able to respond (don’t jump into action without a plan/total picture). My second concern would be not injuring any pax. Third would be damage to aircraft systems/depressurization, if it’s a critical system its probably in triplicate on modern airliners, and once again the probability of crippling damage would be minimal.
Also I would know what the symptoms of Hypoxia were, and watch for them. It’s a great way to calm down the most rowdy pax.