In terms of lore, I honestly think Ken is actually stronger. It feels like he can compete without really trying. I feel like if he was as focused as Ryu he would beat him. I feel like Ken also had to go through more struggles to get through his training as a foreigner in a foreign land. Ken also has a family which he needs to protect and come home to. That gives him more strength than a lone wolf like Ryu in my opinion.
But beyond those lore perceptions, Ken's techniques in the games just seem stronger. There's also the fact that when they tried to play many CPU vs CPU of SF2 Ken vs Ryu matches, Ken came out ahead too often to be considered random. This was actually due to a mechanic called "Core" that Capcom had tried to put into SF2 to make it so if you got hit while performing a special move you would take increased damage. Ultimately they decided against including the mechanic but they forgot to remove from Ryu's Dragon Punch so that's why he is weaker and lost more fights. Of course this is only SF2 but still... Here's Okamoto explaining it:
