A huge mob of IG.
Interesting article here http://pathfinder-devilin.blogspot.com/2011/12/40k-6th-edition-what-needs-fixing.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+blogspot%2FCFmrO+%28Pathfinder%29 about 'improving' 40K. Suneokun's ideas about the FOC and adjusting army lists for flufftastic reasons are good, IMHO.
The way he mentions the manner in which (and he uses the blood angles as his example) an army chooses to deploy itself is rarely as contrived as the normal 40k pick up game seems to make it.
For instance: the majority of campaigns against the Great Enemy are fought against the heretics and renegades of the traitor guard types - the Vraksians, Blood Pact, Sons of Sek and all of their jolly bunch of chums. Now, as hobbyists we do have options for heads and other parts to build these armies out of, but considering that this is the one major fighting force in the 40K universe that all of the other armies are fighting all of the time, they are supported only by Forge World ?!?! Shurely some mistake.
Oh well, getting back to the point: It is entirely conceivable that armies are organised in ways other than as presented in the various codices. For instance the Black Guardians of Ulthwe are a 'professional army' without being aspect warriors. This could be reflected, not just in the fluff and the statlines, but in the way they are organised as well; armed and equipped as soldiers (equal to the aspect warriors) rather than militia (vanilla guardians). They might have snipers, D cannon teams, jump troops, squad special weapons, odd squad sizes (squads of five that can mob up and split off as ordered during the game).
Most SM are described as being deployed as a 'strike force', deploying from a space ship, rarely something as grandeous as a battle barge (which seem to be reserved for multi SM Company actions). So the SM Strike Force Commander has a limited amount of material with which to complete his mission. He can get fuel, water, food etc from worlds en route. He may be able to request munitions from the Deptmento (although I can see that ending in tears). Essentially he has a very small force (of admittedly, super human genetic freaks in powered armour), with a finite amount of material with which to complete his mission. Once he is out of special 'won't-fit-IG-ammo' rockets for his missile launcher, his bolt is blown, game over, pack up and go home. They may be super tuf, but they are not war winners. If you want some individual dead, one or two objectives taken, psychos for a forlorn hope for something like that, then yes, the SM are your men. If you want to win a whole war and hold the ground you capture, you need the IG.
The Departmento launches IG armies into campaigns, usually with a huge logistical tail. There are departmento labour corps to conduct civil engineering works and supply and maintain the fighting end. There are supply chains stretching back tens of worlds, all focussed on the eventual aim of allowing the pointy bit to do its job. All of this is a huge effort and does take a long time to set in motion. In galactic terms it is not strategically agile (although this does not mean that individual IG armies cannot locally be agile; just that they are tied to their supply chain). IG win wars. If the High Lords send a serious IG army to your world, they will win.
So there is your campaign basis: the SM player is not allowed to replace any casualties he takes. At all. The IG player, like the 'nids, always has the endless wave replacement rule going. This 'realism' could be extended to other armies. Although I have no idea how.