When choosing an ultrawide lens for a camera, you do want to consider a few factors:
Focal lengths. With ultrawide zooms, while 10mm vs. 12mm may not seem like a lot of difference, the wider the lenses go, the more those individual millimeters mean in terms of the field of view you're going to receive from the lens. Given that your two choices are identical in this regard, you probably don't need to worry about it. But if you were looking at, say, the Tokina 11-16/2.8 or the Sigma 12-24, it might be a different story.
Format. Get a lens that matches the format of the camera you're shooting, but understand that if you upgrade from "crop" to "full frame", you may not be able to use this lens, and will have to purchase another--far more expensive--lens to do the same job on full frame. This is simply the nature of the beast and crop factors. Since both of the lenses you have listed are "DC" (crop) not "DG" (Sigma's designation for full frame), you should be good until you go to a 5D or 6D body.
Optical performance, particularly in the corners and wrt chromatic aberration and distortion. Ultrawide lenses always come with some form of distortion, and typically can become soft in the corners, and exhibit vignetting or chromatic aberration. All of this is easily fixable in post, but takes time to do so. The higher the volume of images you want to turn out, the more time (money) it takes to do so when lenses exhibit issues like this. In this regard, the lenses are pretty equivalent, although the f/3.5 version will exhibit more issues when used wide open at f/3.5 (something the 4/5.-5.6 version can't do). This is pretty typical. But stopped down to the same settings as the slower version, the performance is pretty equivalent.
So, basically, the only advantage the f/3.5 version has over the 4.5-5.6 is the slightly wider max. aperture. But it's not that much wider, and f/3.5 still isn't particularly fast. Most folks prefer an f/2.8 or faster lens for available light shooting.
Things you typically aren't going to care about:
Autofocus speed/accuracy. Because you're shooting interiors, which don't move particularly fast, and lower light levels, you're most often going to be shooting on a tripod. You have the time to manually focus, and using the LCD with magnification can be more accurate than autofocus. The HSM focus motor feature is not needed for speed or silence.
Maximum aperture. Again, because you're liable to be shooting on a tripod, with good technique, you might even want to stop down for sharpness and just use longer shutter speeds. A wide fast maximum aperture is really only going to be useful is someone is handholding, and even then, you're probably going to need f/2.8 or faster, and neither of these Sigma lenses would be appropriate. A lens like the Tokina 11-16 f/2.8 or a fast wide prime might be more appropriate.
What really makes or breaks real estate photography, however, is the lighting. So it may be worthwhile to consider getting an even lower-cost lens, like the EF-S 10-18; and throwing money towards lighting gear/training instead.
