Earth Kingdom View
Yes, the Earth Kingdom have tough soldiers. Anyone will be awed when they see a boulder-sized rock coming their way. Many firebenders saw a rock in their last seconds. Mighty earthbenders stood their ground for a century. They are also very, very, very stubborn. They won't yield to their enemy.
That is a perk in the military.
Their enemy, the Fire Nation, also sustained an attack for a century. This means that the Earth Kingom is not strong enough to drive the firebenders away. It has to do with their fighting style. They let the enemy strike first, absorb the attack, and counter-attack if they have the chance. Neutral Jing as King Bumi says. However, that strategy is poor in dealing against the aggressive armies of the Fire Lord. The results, the Fire Nation chipping at the edges and the inefficient and corrupt government at the inside. The Earth Kingdom is also not as united the Fire Nation.
Fire Nation View
Now, we all know that the Fire Nation and their military might are mean, bad and almost invincible. Heck, their navy is invincible. The Earth Kingdom has wooden boats, and the Water Tribes still use waterbending to move their boats. The Fire Nation has hulking iron boats, some so large that they are larger than an entire village (like when Zuko was in a Southern Water Tribe village, and his ship is considered small). Their land army is also impressive. Though almost always outnumbered by their enemies, they always win because of their fighting style ( almost no defense, always attack) and their superior technology.
Their style of attack makes them vulnerable. During the Day Of The Black Sun, a small strike force of Southern Water Tribe Warriors, Earthbenders and Team Avatar penetrated that far into the Fire Nation. It could be argued that Fire Lord meant it to be a trap, but the forces sent against the strikeforce could have destroyed the said strike force immediately. They easily outnumbered the small invading group 10-to-1. It shows that they will have a hard time defending their motherland. This could be offset, however, by their technological prowess.
VerdictI could compare the Fire Nation and Earth Kingdom like the aliens and humans in Halo Wars. The humans have inferior technology than the genocidal aliens but ( and unlike the Earth Kingdom) they utilize the alien technology they salvaged and used this against the aliens (very waterbending style). I would say that the Fire Nation and the Earth Kingdom are equals. The Fire Nation has a higher form of technology and has more competent generals (Although they are ruthless, even to their soldiers. Remember Zuko and his scar.) The Earth Kingdom has more men and are good defenders, creating unpentrable cities. But my final verdict is still the Earth Kingdom, I live in there
If I feel up to it, I'll write my first fanon tomorrow and I will be changing the title into Avatar Yangchen: The Lotus Trap because I feel it is a better setting. The civil war of the Water Tribes will still be included. Don't get your hopes even then. I might get lazy.