Man, this was a tough one. It would have made it easier if I had one of those handy switchboxes. Of course, that wouldn't be very audiophile-like now, would it? Anyway, here goes.
Both speakers were demoed in the same location (refer to RT12 vs. RT800 thread), and they seemed to be pretty happy there.
RT800 vs. RT55
The first thing that I noticed when I switched from the RT800's to the RT55's was that the sound opened up a bit. The soundstage expanded a little and there was more seperation between instruments. The sound seemed to be a little less veiled than the RT800's, so I was getting a little more detail. I also noticed that the higher frequencies were more pronounced. I'm not really sure why this would be the case, though. The only thing I can think of is that these observations were the result of the 1dB efficiency difference between the speakers. All of these observations could have been the result of the RT55 playing slightly louder than RT800.
Even though the soundstage was a bit larger, I felt that the RT800's imaged better. I also got a more stable center image with the RT800's.
The biggest difference between these speakers, as you might imagine, is in their bass output. Predictably the RT800 had fuller and deeper bass. However, it was sloppy in comparison to the RT55's nice, tight bass. As my 7th grade English teacher liked to say, "Quality, not quantity." I'll trade off the greater output and depth of the RT800 for the more accurate bass of the RT55.
So which would I pick? That's a tough question. I think I would take the RT55. However, if someone didn't have a subwoofer I could see them making a strong case for the RT800's.