Okay, oversimplifying fag.
One decided as a kid that the best way to save his country is to kill his dad, then to ensure what's best for Japan, join the OTHER side, fights for them, and hopes to change the status quo that has lasted for hundreds of years by being just an example, not even aspiring to lead or create factions or rebel from within.
He mostly fights and goes in dangerous rescues in the beginning, hoping he will also die.
Also, with minimal training, gets to pilot the best mech in the series and perform crazy shit.
Refuses to give up on ideals, even to help his friend or country.
The other barely survived a disaster noone knows about, is saved by a man, and his only emotion he actually feels and can act upon is helping and saving people.
No grand goals, no grand meaning, just help people in his sight, without caring who are how they came to be.
And you say plothax, but for the biggest time, he just gets either slashed at two (Fate) or can barely keep up. And even with his ability, he can still get killed/slapped around (hell, he could die from a grenade).
And Shirou does bend or give up his ideals on certain situations to help those important to him.
Also, fucks his women.