On Thursday, I'll be running a six or seven person foray into my megadungeon. (Or perhaps another adventure. Not likely, but I haven't ruled it out.) I considered using 4e, because it's been on my mind a lot lately and I've pegged it as a good system for one-shots, due to its consistency. Ultimately, I decided that Swords & Wizardry would handle the group size better, and I've been looking for an opportunity to introduce this gang to neo-classical play for a while anyway.
Another consideration is that we're going to have a brand new player at the table. (The younger sister of the boyfriend of an old regular from my high school group.) I've already sworn off introducing new people to 3.5 D&D. Despite my occasional frustrations with it, I like the system, but I've been playing it on and off for nearly seven years now. I've read all the core books, cover to cover, multiple times.
New players don't have that experience, and trying to introduce them to the idea of roleplaying and actually play the game while also explaining the system to them is an exercise in frustration, even if they are interested in its intricacies. More often, they aren't. The last few times I've tried to run 3.5 with people, it's turned into tedious frustration. Boytoy had decided roleplaying "wasn't his thing" partly because of his lack of interest in dealing with that system, which I'd originally used to introduce him to gaming.
4e isn't quite as convoluted as its predecessor, but it's still a decision heavy, rules-mastery focused system. I'm comfortable running it (and, presumably, playing it) but a large part of that comfort is based on that same experience I have with 3.5, and such systems generally.
For new players, it's just too much hardware. They don't need all that stuff to "get" roleplaying, and if they're not interested in it, it just gets in the way. If I suspected she'd be a rules junky herself, and we were starting up a proper campaign, I might consider it. But for a one-shot, and a player I know basically nothing about? Much better to go with a system that gets out of the players way and puts the heavy lifting on me, rather than the book.