False. Yang is strictly optimal. You get a better industrial base for whittling down key rivals, an absurd expansion rate and the ability to support larger horizontal populations. If the map is large enough that a Diplomatic Victory is more than a conquest win deferred, you want Yang.
Dude, what crack are you smoking? Lal gets 2x votes! If he gets the Empath Guild, which he jolly well better be working towards for a Diplomatic Victory, that's 4x votes. And he's got population relaxation and extra Talents. Don't know what drugs you're on. Sounds like you never try Diplomatic Victory.
The meaningful question is what happens when humans face humans.
Not really. The game is too long for it to happen very much, so it's just an accident of people's historical play styles. Who's got the patience to resolve this question among uber-equals? Just how many human player games are you basing your conclusions on?
Another problem is, are you going to play a game with
strictly humans? A full complement of 7? Then the game is determined by the quality of the players, their political deviancy, and their starting positions. Factions may influence it some, but you're not exactly benchmarking faction capabilities in such a game.
If you don't play the game with strictly humans, and you've got 7 factions in the game, then the AI factions are "food." The game becomes about who can eat the AI food faster. Starting position matters. Do human Lal and Yang start right next to each other, or on opposite sides of a large map with AI food in between?
I've figured out a way to benchmark this without human competition though. Players try Yang or Lal, according to their preference. They are required to win with Diplomatic Victory. They record their best times, shortest number of turns to win. They play on the same size map with the same settings, but it's randomly generated each time. Players publish after action reports. Probably a lot of debating ensues about "why" so-and-so won, and whether it really had much to do with the faction or being parked next to the Monsoon Jungle or whatever. But eventually, enough games would give the answer. Unfortunately, even that is a large number of games and might be considered tedious to know "for sure." Still, it is a somewhat interesting question as when I replayed SMAC a few months ago, I never tried any Diplomatic Victory. Haven't done that sort of things since many years ago.
Moderator Action: Warned for trolling
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I beg to differ. I put an incendiary comment up at the top, talking a little smack. Trolling is when you're trying to ruin someone's forum and not add anything constructive. Be that as it may....
My 1st attempt didn't go so well. I played Lal, standard size map, standard map settings, Look First and Do or Die rule changes. Transcend difficulty. I landed in a rough neighborhood, next to both Miriam and Yang. So I eschewed democratic principles and allied with both! Zakharov was nearby and belligerent, declared vendetta on me immediately. So I told my allies to go kill him. Yang got tired of being my ally, but not that many turns later, got more tired of Yang thrashing him and begged for my help. I went Fundamentalist to keep Miriam from irritating me, and helped Yang stomp Zakharov pretty far back. Which I figured I needed to do, because he had found the Monsoon Jungle. I wanted to be over there, but I made the mistake of letting Yang take the cities. Didn't mean to do that, but it's how the combat kept turning out. Then Yang blew me off as an ally again and I didn't have access to that theater of war anymore. Yang and Morgan eventually killed Zakharov off, so, I figure I at least eliminated a threat and kept the Monsoon Jungle a disputed area.
Deirdre had been left alone on another continent and took umbrage to my Planned economy. She irritated me with occasional troops showing up, I stole things from her. Mostly I ignored her and grew my pop. I also sent out ships early and popped a lot of unity pods, making a lot of money. Miriam for some reason never expanded. Seems she settled her first 2 cities so far apart, with myself between them, that she got confused and didn't know what to do. I had been building a secret project, expecting to get Centauri Empathy and get the Empath Guild, but my Fundamentalist phase had kept me from getting the tech I needed. Ended up building the Virtual World, which wasn't useless but not what I had in mind. Brought my troops home and went Democratic. Deirdre got Centauri Empathy first and was building the Empath Guild. I got worried and started focusing all my effort on getting probe teams up there, but my own research kicked in just before I was going to steal it. Built the Empath Guild no problemo.
Then I noticed, my pop wasn't growing anymore because I was food limited. I needed to terraform and I just hadn't put the time in. I had a few big size 8 cities and a larger number of size 4 cities. I started thinking, hmm, I have overwhelming votes right now but I can't win this until I get Mind / Machine Interface. So that means I need a lot of research. So I built network nodes, energy banks, and bio labs. Didn't really build formers. A lot of my cities had forests around them, figured I'd get Tree Farms eventually. Things were going ok when...
...Deirdre made a surprise amphibious assault on a large, fairly isolated, weakly defended city. She actually took it. That was annoying and would have been totally preventable. It's so easy to get bored, I just don't feel like pushing troops around anymore. Do that too long and someone comes for your head. I felt like I had already done a lot of garrisoning, and indeed for any landward attack I had. I just didn't take Deirdre that seriously, I didn't think she had it in her. I quit because, well, even though I'd clearly retake the city eventually, it was boring to worry about it, and I wasn't going to get a stellar time out of this game.
The moral of the story is, I had to play like a reactive chameleon at the beginning, to make sure I didn't get trounced. Then once I got some power, I realized the marathon run was getting to MMI. It's such a long ways away that maybe conquest really is the only option. I thought about just wiping Deirdre out, but I wanted to try to do it "Lal's way." I guess if you're going to do that, you have to be a bit more defensive than I was.
. They are on a different continent at least. 
Certainly enough to give a good indication of the relative strength of the factions.
