Riggswolfe said:

Both girls in my group love playing Space Marines.

ItsUncertainWho said:

"You shoot your lasguns. They all miss. The bad guys shoot their lasguns. They miss. Next round: everybody misses, and no hits in the next round."

Lingula said:

And the round after that the bad guys give up because they are outmatched!

You also forgot the montage build sequence.

Yes it does warrant a new game mainly because the imperial guard and deathwatch space marines are two different military groups in the Imperium. Thus the playing style begins to vary. As well, the roleplaying aspects of the two would turn out drastically different.

One of the major differences between roleplaying Only war and Deathwatch is in the scale, in Deathwatch
you alter the course of an entire salient, possibly even the entire crusade. As a squad of veteran guardsmen, you
alter the effect of a battlefield/warzone. As previous people have pointed out, most war movies do not fit well with
the role of space marines on the battlefield. Where as they fit perfectly to the role of the imperial guard.

For one, I can't wait to run a campaign as follows: (For arguments sake, lets say our OW squad is drop troops)
Imagine omaha beach-like situation. Now it is only a world being invaded by the Imperium. Before anything, the
Deathwatch marines are sent in to try and assassinate the enemy general/warmaster/big bad guy. They have to
preform this task (and possibly several side quests) in a fixed amount of in game time, before the Imperial navy arrives
in force. Then our OW squad moves in on their Valk, dropping behind enemy lines, their job is to take and hold a landing
zone/destroy communication stations/ disable orbital cannons/ capture or destroy bridges or so on. Their job will get
easier/harder depending on how well the Deathwatch team performed.

Then with a good breach head against the enemy, the imperial army moves in and start the slow job of retaking the entire planet.
Thus, the Deathwatch is no longer needed, and they can carry on with their own jobs, on other planets and other war fronts. They are
way too scarce and valuable to remain active in this war front. Whereas the OW squad will remain and continue
with the war effort, and have several more missions to do before the planet is secure. Also they will their missions will effect how
well the war effort is going, thus impacting their logistics rating.

Well, that is just what I had in mind.

I'd personaly say the relationship between Only War and Deathwatch is pretty much identical to that of Dark Heresy and Rogue Trader. Basicaly Space marines and Rogue Traders have a lot more freedom to act in the ways that they want to when compared to Imperial Guardsmen or low level Inquisitorial Agents. Only War and Dark Heresy give you a look at the lower levels of Imperial society and maginfies the struggle for survival. Whar could seen to be easy meat for a squad of Space Marines or a starship can appear as insurmountable odds to a squad or two of Imperial Guardsmen.

Only War should be an oppertunity to show the uncaring nature of the Imperial organisations. Limit their tools to get the job done, hand out almost sucicidal missions, introduce incompetent officers to lead them, roll out the tyranical commissar to offer them the choice of victory or death etc. Make them realise that the only thing there is between their character and a messy death is their squad mates.

Kahadras

The personalities that make up the Imperial Guard are as infinite and as varied as the human race, the possibilities are as limitless as human nature itself. You don't get this with Space Marines, at least not to the same extent. The regiment will play a large roll in shaping the identity of character but your much more likely to find the full gambit of human emotion in the Guard than you are in the Astartes. With pride, stupidity, determination, cowardice, anger, fear, greed, heroism, jealousy and even treachery colliding within the same squad, let alone the same company or regiment. I think the potential for diversity will be a major attraction with sweeping campaigns featuring Armoured Regiments of every stripe, Storm Troopers, Rough Riders, Drop Troops, Aeronautica (Anyone fancy Voxing in an airstrike on that Ork compound?!) not to mention all the various worlds and battlefields you could fight across, perhaps requiring a Battlefield Compendium might be cool?

I don't want to slag off Space Marines (I'm not that dumb!) but Imperial Guard are just better and offer so much more potential for RP, the chain of command is an excellent DM tool, if you think its an obstacle to RP then thats what it'll become, but really its another avenue for plot hooks. Such as your squad being given all the worst missions because your CO dislikes one particular grunt in your squad, I'd love to try out that dynamic. I'd also try putting my grunts in space in a Naval Battalion having them boarding ships, space stations and orbital defence batteries as well as putting down riots (in fact putting down a riot plantside could be an awesome intro to a defensive campaign)

DW

The real difference is the human face on things.

The Adeptus Astartes are His Angels of Death, they shall know no fear. A Space Marine's job is to take on the biggest and nastiest beasties in the galaxy, and use his amazing training with the best weapons possible to triumph. He hates the mutant, the heretic, the xeno. He knows his role as a living weapon, and thrives in that role.

Imperial Guardsman are conscripted en masse. They're stuffed into flak armour and handed an M36, and then they're shipped across the galaxy to fight things he doesn't understand. He misses home and his family. He'll likely spend a lot of time in a muddy trench. When the Carnifex shows up, he'll probably piss himself in terror or run forward because the alternative is a bolt round to the back of the skull. Guardsman are average joes: farm boys and hive orphans and manufactorum workers. They want to smoke lho-sticks and eat something other than corpse starch for once. They want to spend a night with the ladies in town. They probably want something thicker than flak and faith to keep himself safe from the claws and bullets of the enemy.

It's the difference between Superman and a kid taking ROTC, except they're both fighting the same bad guys, bad guys that can and do kill Superman. You can tell a lot of stories with that. An Astartes sees a populace turn to secession or The Greater Good, and sees only weakness. He kills them and probably feels little. A Guardsman sees a secessionist and he understands that it really, really sucks to have the only Ecclesiarch he's ever seen preach about the good of the working class and then have an assassin sent after him by the very Imperium he should love. He hears about the Tau offering a life based on working together, where all toil equally instead of having nobles spit on him, and he probably sees why that's a good idea. He probably thinks it beats having a Commissar point a bolt pistol at his back all day!