Hi, I'm new here though I've been active in a few other ST forums. I registered just to vote. And my preference is... well, there is a hint in my username...
yes, SPOCK. Spock was hotter than Kirk in TOS and Spock was hotter than Kirk in this movie.
To me, it is simple. When it comes to hotness:
Leonard Nimoy > William Shatner
Zachary Quinto > Chris Pine
Spock as a character > Kirk as a character
Spock's looks (in both versions) are a lot more striking, exotic and captivating than the conventional looks of the Kirks, and as a character, he is just so much more intriguing... His logical, serious persona is such a refreshing change from the gung-ho action heroes, and of course the mysterious and unapproachable are always particularly attractive... But the sexiest thing about Spock is really all that smoldering passion beneath the cold exterior. Kirk had many flings and flirtations (which, frankly, bored me), but whenever Spock fell in love, he was passionate and romantic in the way Kirk never was or could be. To whoever said that Kirk would be more of an animal in bed... I don't think so!!! Spock is like a vulcan (pun intended) that can explode when you least expect it. All those pent-up emotions... Not to mention that he has greater strength and stamina than humans, as well as enhanced senses...
Uhura Prime said it (or sung it) best:
Oh, on the Starship Enterprise
There’s someone who’s in Satan’s guise,
Whose devil’s ears and devil’s eyes
Could rip your heart from you!
At first his look could hypnotize,
And then his touch would barbarize.
His alien love could victimize…
And rip your heart from you!
And that’s why female astronauts
Oh very female astronauts
Wait terrified and overwrought
To find what he will do.
Oh girls in space, be wary, be wary, be wary!
Girls in space, be wary!
We know not what he’ll do.
As for Pon Farr - Vulcans don't have sex once in 7 years. D.C.Fontana said that this was a misunderstanding by the fans. They have sex any time they want - Pon farr is only the time when they HAVE to or they'll die. Come on, you did not really think Amanda and Sarek did it just once in 7 years? LOL
Now in TOS, it is not that Shatner was not good-looking (when he was young) or charismatic, he was...(although he could also be annoying) but he paled next to Nimoy's Spock. He was just mesmerizing, and I couldn't take my eyes off him. Sure, it helped that he had a great body in that tight uniform.

but mostly it was due to talent and charisma, his powerful presence and a hint of suppressed emotions underneath - not to mention the episodes when Spock would lose it and we got to see them surface in all their moving and frightening intensity. To my surprise, although he looked old in the movies and his looks were not what they were in TOS, even in Undiscovered Country I felt flashes of the old sexappeal when he was angry and hurt.
Zachary Quinto is a rare example of an actor who has all those qualities... I think he is much handsomer than Chris Pine - Chris is a lot more likeable than Shatner (and better actor) but he is not my type, looks-wise he is too much of a conventional pretty boy face. Zach has much more striking, unusual features, with the most expressive eyes, the kind of face you just notice. But more importantly, he has this incredible intensity and amazing, somewhat scary sexappeal - as Sylar, as Spock and even as himself (he can be goofy and adorable in RL but you still feel flashes of something unusually serious, intense and introverted in him). The moment he appeared on Heroes, I could not see anyone else, even though there were several gorgeous men there, and some of them might be conventionally better-looking, but none of them made me hot and bothered.
I was concerned how he is going to look without sybrows, and I did not like the airbrushed promo pics - but oh my God was I wrong - he looked gorgeous with the Spock eyebrows and everything!
I don't know what is wrong with people who say that his Spock is "asexual"?!

Come on people! He has so much smoldering passion beneath, I get goosebumps just looking at those burning eyes!
I wonder if Quinto's Spock is not as popular as Nimoy's Spock because Pine's Kirk is handsomer than Shatner and because Spock is more emotional in this movie and he's already got the girl?
In TOS, Spock was this controlled intellectual who had these seething emotions running under the surface. To me, that was sexy as hell, both because he was smarter than anyone else and because you wanted to hug him to get those emotions out more. In XI, Spock's more emotional because of what happens to his planet and because he's younger. Uhura gets to hug him instead.
He isn't? That is news to me. From what I've seen in other places all over the Internet, Quinto's Spock is far more popular with girls/women than Pine's Kirk. ,On IMDB Star Trek board, there is a bunch of "Spock is sexy" threads, and there is a similar thread to this one: Who is sexier, Kirk or Spock?
http://www.imdb.com/title/tt0796366/board/thread/137661319?p=1 and Spock is winning by a landslide, the last time I tried to count, it was something like 55:10. Some of the women who replied are TOS fans who have been crushing on Spock for a long time, but most are new fans who were not familiar with Trek before and fell in love with Spock in this movie.
Also, from the comments - and Youtube videos - I have seeb, his scenes with Uhura are a big part of the reason why many of the girls/women find him so hot.