Yeah, regardless of what mode we do, I'd have to put my vote for dark. At least half the fun for me is predicting how enemies will move, based on the map layout, unit composition, and their last known position, and then placing my own units to have fields of fire on where I think they'll be. Light ruins this by making long-term thought almost an *hem* afterthought; if you always know where everyone is, all you have to do is guess what they'll do next turn, and the outcome basically comes down to who has better initial positioning at the start of the turn, rather than who planned better, because with total knowledge of everything but enemy thoughts, you can typically nail down every possible movement for each unit and plan your own movements to counter that. That isn't tactics, and it certainly isn't exciting. If you're in a situation where they know exactly where you are, and they'll have the advantage in every possible encounter, the only things you can do are a) something incredibly stupid that may be unpredictable enough to pay off, or b) pray that the opponent missed a counter. Neither is skill, both are luck.
I'm trying not to bring realism into this, for obvious reasons, but this is a game based on squad tactics. A large part of that is dealing with the aspect that you should only be able to know what the enemy is doing or where they are if a) you predict their movements correctly and b) if you can see them, or have seen them fairly recently and can give a reasonable approximation of where they are now. I (and I assume most other people who do so) prefer dark because you're basing your plans on where the enemy units were, what they are, and what you think are the most reasonable things for them to do. I play by looking both sides and figuring out the best movements, particularly in regards to longer term strategy (taking defensable rooms, setting up lines of fire, capturing objectives as safely as possible), and then what I would do for each side to counter those movements. Once I've got something worked out that should let me outplay the enemy (usually by not taking the first or second obvious choice), I lock it in. I don't think that players should just be able to ignore that aspect of the game. I don't care that light exists, but I absolutely detest playing it.
Just my opinion, and I'm sure someone disagrees with it, but I thought I would put it out there before we get too far into this.