Buge is sort of right, but I'll give some more detail. They are significantly different, especially in Super Turbo. They were indeed identical in World Warrior but that proceeded to change with each game.
To highlight some uniqueness
Ryu:
*Better fireballs (faster and recover sooner)
*Hurricane kick knocks down on hit
*Jumping MP can juggle (into super even!)
*A grounded overhead that comnbos into sweep
*An advancing punch
* Fierce Kick good anti air
*Red Fireball, which recovers slower but can knock down if close.
Ken:
*Better Shoryukens. His Jab DP is safe on block and his fierce hits INSTANTLY (0 frames). Also has more horizontal range.
*Hurricane kicks combo but do not knock down
*Fierce Kick is an excellent poke
*Medium Kick throw is an awesome hold style thriw (untechable) that leads into awesome mixups
*Has 3 special kick commands (qcf kick, DP kick, HCF kick) that can be used in combos or sometimes as an overhead.
In practice...
Ryu:
Awesome fireball zoner, better overall solutions to most problems. He has tighter anti air. though it might not sound that way, but his jab DP is better anti air then Ken's Jab DP (it has head vulnerability and is better for ground fighting). His Jumping MP juggle is damaging and also good anti air. He has very respectable mixup, though simple and can end most situations with a knock down. He's overall the better of the two.
Ken:
Less good at fireball keep out and anti air, Ken is better at ground fighting and aiming for throws. Shoto Jab DPs can even hit sweeps and with Ken's being mostly safe, he can spam the hell out of it. His knee bash grab leads to one of the best mixups in the game as he can choose between crossing under you or not while you recover, or safe jumping in with jumping lk. He has a lot of flashy trick shot ocmbos with his funky kicks too!
In the end they play every different, even if their changes don't sound so dramatic.
