Its kind of hard to make a decision based on no information.
If the marines know they are likely to face big mean alien/robot/marsupial things that like to get as close as possible in a cramped spaceship/building taking along a fast firing shotgun that just fills the corridor with nasty death very quickly, without damaging the spaceship itself too much, might seem like a good idea.
But it wouldnt be much fun for the alien/robot/marsupial and neither would a Q2 slow firing super shotgun thing or incredibly accurate explosive rounds of extreme pain +3.
Somewhere in between with a reasonable medium range weapon might be a good idea, but most shotguns that are good at medium range just get nastier at close range (CS/Firearms/Q2DM/redfaction and gloom to an extent) and I'm assuming that there will be 'long' ranges, which is unlikely given the q3 engine and screenshots available so rifles would probably have to be most effective at 'medium' ranges anyway.
But if they have omgsokewl flamethrowers anyway...