I will usually try and run a full run if I see DPS who are below the cap for the dungeon. I have tried the method of asking people's opinions, but then you get those who are, as previously stated "brain dead", or you have differing opinions and then people get angry when you don't agree with their side. As far as gaining XP goes though, I will usually run longer dungeons for the DPS since their que times are so bad. It also depends on how good the players are at their roles... if the players are really good, I will be more willing to do a longer run. If they are doing terrible and won't accept any constructive criticism, I'll try and finish it quickly.

I don't do many speed runs though in either scenario. I'm not superb at them and a lot of DPS and healers aren't either in the lower level dungeons.