Well, I've actually got a passive display, and I can tell you that what I'm seeing with my eyes is most definitely NOT 540p. I can't count pixels, so I'm not sure if it's a full 1080p, but it's absolutely not half resolution, naysayers be damned. Turns out your brain's actually pretty good at assembling what your eyes see into a cohesive picture. Who knew?
The only thing I notice, because I'm trained to look for it, is some slight aliasing around some high-contrast edges. Granted, this is also a monitor viewed from only around 30" away, I'm sitting closer than is recommended, and most users might not even see it, or notice, or care. The aliasing is caused by the fact that the two pixels of the image aren't actually right next to each other like my eyes are telling me they are. The aliasing is not nearly as bad as, say, a video game with no AA turned on. I can tell the difference in AA levels even when playing games in 3D, which should tell you something, since it's still pixel-for-pixel and I can see the difference. Definitely not half-resolution, or else it would look a LOT worse.
Let me put it this way: I can see the difference between a 1080p YouTube video in 3D, and a native 3D Blu-ray being played directly from the drive. The BD is absolutely clearer and sharper, just like it is on normal 2D content. If it were really half resolution, I shouldn't be able to see that much clarity in two different 1080p streams.
There's a big difference between what's "technically" happening and what your eyes actually see and how your brain interprets it.
As for the original topic, I would agree that SBS is better, when you're talking half-resolution videos like YouTube. The eye seems to key more off of vertical resolution than horizontal, and T/B images seem overall softer because of the loss of vertical resolution than SBS images. If you're talking full-frame images, where the original 1920x1080 images are stored in their entirety, then it doesn't make any difference.
Welcome to Rivendell, Mister Anderson.