All things equal, I would expect the infected to have the edge, though it is highly dependent on spawn combination and location. That's why each time plays both sides, though.
From what I've heard of high end competitive matches, this is simply false. My experiences even in public matches lead me to believe that survivors really do have the edge. It's not uncommon in competitive matches for the survivors to make it through on every map (minus, say, death charges/instant deaths and the Parish finale).
The main obvious problem is that the survivors have too many items. The scoring system is based around the idea that the survivors won't make it to the end of the level. As stated in commentaries for L4D1, this is the developers' intent. But for every chance the infected have to damage the survivors, the survivors find health items laying on the ground. Med kits everywhere, defibs everywhere, throwables everywhere. Any damage the infected can do can be totally negated. Most times when a team doesn't make it, the other team is much, much better.
A really good survivor team can (and will) totally shut down infected attacks. Shooting hunters out of the air. Deadstopping hunters. Shooting boomers through walls. Hordes are almost useless with the addition of melee weapons. Not that they did much anyway, when half the survivors have pipebombs at any given time. Frag rounds are a particularly bad offender as an overpowered weapon. They can stumble witches, or knock any special infected away from what they're currently doing (stumble chargers, hunters, etc).
Everything is geared towards the survivors. There are sounds that play for the survivors (music cues) that tell them what infected are currently coming in. Before they spawn. All infected except for hunter give their position away constantly by making weird sounds. Burping, screeching, etc. Tanks are a total joke for a good team. They can either light him on fire and then run (he can't catch up), or just lead him around and shoot him. As they're probably green, they'll most likely take one or two hits before he goes down.
Witches are even worse. It was ridiculously easy in the first game to oneshot the witch. Now it's even easier, since the wandering witch takes longer to get set off. Not only that, but the new witch takes 6-7 seconds longer to kill someone.
Halfway through each campaign, the survivors start getting upgraded weapons (assault rifles, autoshotguns, etc). The infected? No upgrades whatsoever.
The point being, even in competition (sponsored tournaments, etc) where all extraneous health items are removed except for one or two sets of pills, there are only rarely tier two weapons, and many of the new survivor items are removed, both teams commonly make it to the end of the match. The skill curve isn't anywhere near the same for survivors or infected. You get much more benefit out of being a better survivor than being a better infected.
None of this really applies at low skill levels or when one team is drastically better than the other. Has anyone been in games where everyone on both teams made it out most of the time? Where any time you got in a good attack, there was a medkit spawn around the corner? Levels like the Parish 3 (I think) where at the car park, you might have ALMOST killed the survivors, finally. They have no health items, they're all limping, one's dead, and then they get to the ambulance. There's now three extra health kits, and two sets of pills. Almost all your work is now totally negated. It's like the level has started over.
Each team gets to play both sides. This makes the game fair. It does not, however, make the game balanced. The game will only be balanced when for an evenly matched team, there's an even chance of stopping the survivors. Currently, that balance is out of whack, and the survivors usually make it the entire way to the saferoom. Even if they don't make it, it's common to get 80-90 %.