Just thought I'd throw in my 2 cents here: Personally, I've become a big fan of waxless seals over the years, but some brands are better than others. The first time I tried a waxless seal was around 2004 or so, and I used the Fluidmaster waxless seal, which was the only one I knew of at the time. Today, there are MANY brands I know of: Fluidmaster, Fernco, SaniSeal, and Korky.
The Fluidmaster seal works, but I see 3 main problems with it:
1) It may not seal perfectly if you don't set it at the right height when you install it--that happened to me on one of my toilets, and I didn't realize it until I started noticing sewer gas smells later. I think Terry has the right idea in putting it over the horn on the toilet first.
2) It's not stackable, although this may not be so much of an issue since it's design allows a fair amount of latitude in terms of the height it's set to.
3) The rubber rings it uses can perish--I noticed this recently when I removed one of them.
The Fernco seal looks similar to the Fluidmaster, but its 1-piece construction does look a bit more elegant somehow. I have never tried it personally.
The SaniSeal is my FAVORITE so far, mainly because:
1) The Fluidmaster waxless kit can fail to create a positive seal if you accidentally push it too far into the drain, whereas the SaniSeal sits on top of the flange just like a wax seal, so you're practically guaranteed to get a positive seal with it every time.
2) Unlike the Fluidmaster seal, the SaniSeal is stackable. (Though as I say, the Fluidmaster design should allow a fair amount of latitude in terms of height, so stacking may not be a concern with the Fluidmaster, either.)
3) Time will tell, but the SaniSeal looks to be made of a material that won't perish the way the rubber seals on the Fluidmaster do.
4) I like how the SaniSeal holds the bolts in place.
The Korky waxless seal looks like pretty much the same idea as the SaniSeal, and like the SaniSeal, the Korky should theoretically work with ANY flange or floor surface, plus it holds the bolts and is stackable. The main difference is that the Korky uses a combination of red rubber/foam rubber instead of green polyurethane. Again, I've never used the Korky waxless kit myself, but I prefer the looks of the SaniSeal, because the material it uses looks and feels more robust somehow.
Anyone else have any thoughts on this?