I must disagree with the OP, who waaaaaaaaaaaaaay back in the distant past (where there was not only war) claim sisters to be Marine's equals. Sisters are badass, sisters are awesome, sisters can pwn marines sometimes. but they are not marines, they far outnumber marines. If the IG is the army, the sisters are the Rangers, and the Marines are Delta force. Faith is a powerful thing, and the sister's use it to great effect, doing things which might make a marine jealous. But their armor, while may be designed to similar specifications, is not the same. Sisters cannot wear as thick of armor and still move properly, their bodies aren't as massive or build for it. Similarly, I've been to a gun range, if my sister tried shooting a large caliber handgun or rifle it'd kick out of her hands or hurt her, she's not as strong. As such marines use a larger gun than the Sororitas. The benefit of the sisters is they come in larger numbers. Anyone who calls them ******* is a flaming idiot, but calling them the equal of an Astartes, who is defined as superhuman well, both sides can have fanbois/fangurls.

I also would like to see a Sister Superior in my DW game, btw, if only to make the Souldrinker brother uncomfortable.

Actually, both Sisters as well as Marines use .75 cal boltguns. The fluff even had them use the same patterns at times, and the difference between Astartes bolters and other models was always described as weight/size/resilience only, so the difference between DH and DW is a bit ridiculous - but this has already been touched upon and I chime in with the oft-stated notion that it simply is another "scale", another way of telling a story.

Though, yes, they are still not equal, not in every regard. I agree that such a statement is over the top.

That said, I'm actually not so sure if the Sisters are really that numerous as you say. The last solid number we were given in the fluff was 30.000 Battle Sisters in the six big Ordos Maioris for the entire galaxy. If you look at the force composition in the DW book you will also notice that the contingent of Sororitas accompanying the Achilus Crusade is ~2.000 Battle Sisters versus ~1.600 Space Marines.

Huh, that numbers thing is news to me, though as a Wolf, I'm not keeping up on the Ladies' stats too much. I keep an appreciative eye on them, yes, but not so much as I might be accused of snooping. I was under the impression that the standards of admissions were somewhat laxer, and so they could recruit faster.

Interesting thoughts otherwise. I always assumed they play it smart and went after CSM with numbers to overwhelm them.

Well, Sororitas fluff really is somewhat scarce, so don't feel bad about missing the occasional bone GW has thrown 'em. I remember back when Soulstorm was released, lots of players thought that the Sisters were a faction created by Relic just for Dawn of War. Goes to show how famous they are.
This may change, though, Dark Heresy already did a lot to make them more "known" and GW/BL are beginning to release a lot of new Sisters stuff in 2011/2012, sooo ...

As for the size, it may actually well be that the Adepta Sororitas as a whole has millions, if not billions of members. It's just that the Orders Militant are their smallest part, only taking the most able-bodied progena whilst the rest goes off to join the non-militant Orders or the Commissariat or (if they're totally unsuited) the Adeptus Terra. The non-militant Orders are very prevalent throughout the Imperium, though, with their Sisters Hospitallers and Dialogous accompanying many Imperial Guard Regiments or Navy Battlefleets, and the Sisters Famulous trying to extend the Sororitas' influence into every major noble House there is. The usual attrition of the 41st millennium's grimdarkness does the rest at keeping the number of Sisters Militant low whilst those of the non-militant Sororitas are stable or continue to grow. Just look at what the 3rd War for Armageddon did to the Order of Our Martyred Lady: they've barely been able to field three Companies of Battle Sisters after the war was over. New novices continue to trickle in and the ranks are swelling fast, of course, but it's just a matter of time until they get knocked down again.

You're right about the numbers being an important factor, though. They may have comparable gear and incredible fanatism, but ultimately they are simply not as "durable" or experienced as Marines (especially as a Sister spends half the day praying, whilst a Marine spends ALL the day preparing for war), which means they have to compensate this with something else. This may mean that several Orders of Battle Sisters make a joint effort, or that the Sororitas are assisted by the Imperial Guard and/or Navy. Plus, of course, the usual contingents of Frateris Militia and PDF.

That's not to mean that they do not try to attack a Marine Chapter with a smaller force at times, however, relying on advantage by surprise or some other tactical feat. This usually ends badly, though, as exemplified by a bunch of Seraphim deep-striking right into the middle of the Sons of Malice as they were busy doing their Chapter rites. It did not end well.

Dammit, now I really want a Sister in our game. Thanks for the heads up on them. Still I find it hard to imagine people didn't know they even existed!

Heh - an unusual, but not unthinkable combination. It is an interesting thought, what with the characters constantly juggling with that weird mixture of opposing dogma or prejudice (one side thinking of the other as abhuman heretics, the other thinking of the first as fanatic loose cannons who lack tactical insight) and some sort of grudging mutual respect for each others prowess and courage or their role in the Imperium that at least seems to be common enough amongst both groups. Not to mention the drastically different kinds of personalities and "modus operandi" that would be thrown together into the heat of battle. Lots of potential there.

Though personally, a Marine character would interest me just as much, just for a change.

If I was there, I'd happily import my Wolf Scout. Used the "Kill-Marine" rules to simulate a one man behind the line guys. and with a high Fellowship and maxed Carouse and Trained Charm, I'd like to think he's a fun guy to party with..... in both senses!