I take Sergeant Guile because i really like to use his Special moves the Sonic Boom and the Somersault Kick. Beside he is more human compare to both Ryu and Ken, who is possibly 2 of the most powerful character in the Street fighter Saga.
Ryu's hurricane kick goes right through Guile's sonic boom. That's the wild-card in this match.
Turtling Guiles can give Ryu a challenge, but if you're a good zoning Ryu, your fireballs are much faster than sonic booms; simply b/c one is a motion move(fireball) while the other is a charge move (sonic boom).
I've seen some top level playing Guile's smash some top level playing Ryu's though... so it can be done... but on your average day, a good Ryu will defeat a good Guile.