Empire > The key is to get your AT ATs and AT STs to camp both exits of Echo Base, which also nullifies the snowspeeders. Get your vehicles and infantry to target the radar dish turrets first, as they do MASSIVE damage against AT ATs (well it's only 1 tiny bar per shot, but that's much more than anything else the rebels have).
Snowspeeders are hardly a threat to AT ATs, as long as you have shock troopers around to shoot them down with rockets. If they even try to entangle your AT ATs, they will be slow sitting ducks, and can be easily wiped out by 3-4 rockets.
Just prevent the turrets and snowspeeders from taking down your AT ATs, roll up to the entrances, and prepare for uber ownage.
Rebels > Do NOT use the snowspeeders to trip up the AT ATs. By now, lots of imps have shock troopers standing by to shoot you down. The gun is also pathetic against armour. Instead, get the snowspeeders to concentrate on the INFANTRY. Do nothing but massacre the poor infantry, especially the snipers who are killing the turret gunners, and think they are safe, on hilltops.
The rebels need to man their radar dish turrets, and have vanguards to take out the AT ATs and AT STs. Don't let them get close, or you'll have a hard time coordinating an assault on them.
Curiously enough, the shield generator can be safely ignored for both sides. Echo base is the most important CP in the game.