I guess I'll stick to aliens.
I like to think I'm a pretty good player, but I have noticed in the dozens of games of NS 1.03 I've played that marines will only have a chance at winning if one of the following conditions are met:
A. If the Marines use a cheap tactic. (For example, siege cannon through unlikely walls to pin down two hives.)
B. If the marines have a heavy numerical advantage. (Possibly because of the way the economy works, but possibly also because the commander takes up one spot on the team leaving them down one man on the field at all times.)
C. If the Marines have a heavy skill advantage. Look at any marine match that was won in which A or B didn't come into effect, and you will see factor I'm not just talking a minor skill advantage, either, I'm talking the aliens would have to be in complete dissarray versus the marines being unnaturally well organized.
Usually a combination of the above.
Given level conditions in which the marines don't use cheap tactics, don't have a numerical advantage, or don't have a heavy skill advantage they're pretty much toast. Granted, I generally play on 16 player servers, and the way the economy works and better saturation of marines often cause things go a little better on 24/32 player servers.
As someone who commanded a few games and can observe how the battle unfolds from that perspective I've made a few observations. I challenge anyone who disagrees with my assessment to try their hand at commanding themselves and see if I'm wrong here.
1. Marines cannot effectively hold any ground without constant patrols. This is a real problem because marines are generally quite slow and completely reliant on the commander to tell them where to go. By the time they get to a resource base, it's too late. Phase pads are the best solution I've found to getting marines to the places that need defending, but often the marines are too tied down elsewhere.
Most of the time, your marines are busy, so the job of holding down resources falls entirely to your turrets. Against a single hive turrets can be effective because a skulk and gorge cannot survive under fire of a turret... but only if you have built your turrets "just right" and a single skulk doesn't find an uncovered angle to remove your entire turret farm while your marine's attentions are elsewhere. Against two hives, a single lerk or fade can eliminate entire turret farms, pausing only for their regeneration to heal any damage they may have taken between volleys. Teams of aliens take out turrets farms much faster. Against three hives, turrets are mere annoyances: game over.
End result: Since turrets can only be effective at the start, the Marine's abilities to hold down resources without constant marine patrols is nill. I am now under the belief that making the turrets' operation rely on the existance of a nearby turret factory was a mistake.
2. Even with full Weapon Labs upgrades, a skilled marine with a LMG is generally no match for an equally skilled fade due to the fade's higher speed, durability, acid rocket splash damage, and special abilities (regeneration, blink, cloak, ect). Outfit the marines with better weapons, you say? I might be able to drop a few guns, but I can't afford anything else. If your wondering why, read reason #1 above again. In order to make the middle playing field even, I think a fully upgraded LMG using marine should be able to have a fair shot at taking down a careless fade.
3. The Alien's weapons have far more utility to them. Lvl 3 alien weapons are nasty, but their lvl 2 weapons alone have a ton of strategic effectiveness that you can't get out of the marine's mere direct fire weaponry. Acid rockets, umbra clouds and webbing come to mind. The only marine weapon that even comes close is the grenade launcher, and it costs a heap. Again, thanks to #1 above the marines won't get many grenade launchers at all. On top of that, carrying a grenade launcher cripples you at close range and you run the risk of blowing yourself up with it.
Ultimately, the main problem is that the Marines lack flexibility. If there were a few more tools at the Marine's disposal that allowed them to compete - say perhaps spawning with a hand grenade they can lob in an alien occupied vent - they'd be that much more viable.
If you're able to win consistantly as part of a Marine team in 1.03 you have my respect. For me, 1.03 looks like it's a dark day for humanity in the Natural Selection universe.
Burned out gamers point out potential imbalances.
Jaded cynics can't tell the difference.