Well basically I like Triple Threat matches, especially if the participants are all decent in-ring performers (see HBK/HHH/Benoit at WM20 as an example), since in such a constellation, there's just so much more possible in terms of cooperation, different finishing moves/sitations and how they can be solved/broken up again by the third man, more possible surprises without the need to have someone interfere from the outside (see Cena/HHH/Orton) etc...
However, as pure drama is concerned, no triple threat can ever have that same "showdown"-feeling that a pure one-on-one encounter has. It really doesn't matter if that encounter is a regular match, or a Last Man Standing, HIAC or other gimmick match... the essential thing about the supposed showdown, especially at Wrestlemania, is that the build-up to that event has to be as personal and as dramatic as possible... and that really works best when you have only one hero, and only one villain, so you know just exactly who you (are supposed to) support and who not. With a third man involved, people might start thinking: "Well, actually I would've wanted X to win, but now that Y is involved as well, I'm also pulling a bit for him...", and I guess that in such a situation, the emotional involvement just isn't as deep anymore as it would've been in a classic "duel" situation, where you just KNOW that this guy HAS to win, and that guy HAS to lose, or else you'd be mightily pissed hehe.
So as for Wrestlemania - I honestly would prefer one-on-one encounters, especially at WM. I really would have no problem with any sort of gimmick match (well, at least those gimmicks that add to the drama factor, like LMS, Ironman, HIAC or 3 Stages of Hell), but it should really underline that showdown-factor. On other, "smaller" PPVs I guess Triple Threats for the title are pretty fine and a good way to spice things up every now and then. But if you want to create a feud that really means something (see HHH/Orton right now - there would be no point in randomly introducing a third man to that program), you need to focus on the conflict between two individuals, since it's the most elementary thing people can relate to, especially if personal reasons are behind that conflict (again, HHH/Orton) instead of just a "desire to become champion" or such. That is why I personally really feel that the HHH/Orton match will be the better one at WM, not necessarily from a purely in-ring point of view, but simply because the storyline is more personal, and gets people more easily and deeply involved. I expect Cena and Edge (and whoever else, if a third man remains in that angle) to do a great job, but essentially, this angle is about nothing else than Edge somehow "cheating" himself to a title win in the EC, which of course is a "heelish" action, and of course the former champion will be pissed by that action - but it is in no way even halfway as meaningful as the personal conflict that is the basis for the Orton/HHH feud.
So my vote should be clear - definitely one-on-one, especially if you have a decent build-up and storyline to go with it. Triple Threats can be a nice change of pace, but should be second choice, especially for Wrestlemania.