The effect of light, medium and heavy armor

There is still no definitive list, but there's a pretty comprehensive review here.

According to this guy,

• PC has better textures and allows to zoom in closer,
• PC loads faster,
• when playing with a mouse you have minor conveniences like hovering on top of aliens-in-view icons to see hit percentages (I'm playing with a mouse and I've never knew it).
• Important: multiplayer is not cross-platform, so you better follow your friends.

Reviewer explicitly states that he felt no tactical performance gain on any platform.

Tactical view grid should also be classified as a minor convenience - after some hours, you'll be easily telling distances at a glance.

Heavies are your primary damage dealers.

• Their exclusive heavy weapons are simply better than the corresponding assault rifle of each weapon tier.
• They carry a rocket launcher, which allows them once per mission to cause damage to multiple targets (on the highest level they can learn an ability to do it twice). Their rocket launchers almost always hit and always do the same amount of damage, so this is a very reliable ability.
• Most of their abilities further increase the amount of damage they can inflict.
• Their bullet storm skill (which is learned very early) allows them to fire twice per round when they don't move. Measured in pure damage potential, this ability is more effective than the rapid fire ability of the assault (which can't be used on two different targets and which has an aim penalty) and the double tap ability of the sniper (which can only be used every 2nd round). By the way: it also allows them to shoot once and then move out of the line-of-sight in the same round.

By the way: with the "Enemy Within" DLC, the assault class can theoretically out-damage the heavy. The "Close and Personal" skill grants them one free shot per round against an enemy within 4 tiles. This can be followed with "Rapid Fire" for a 2nd and 3rd shot in the same round. "Close Combat Specialist" can give them another free shot during the enemy turn when an enemy moves within 4 tiles for a total of 4 shots per round. But keep in mind that the assault can only use this potential when played in a very aggressive and risky way, while heavies can do their damage while camping behind a full cover.