He's most certainly stronger. Just wanted to say he's also more talented. Technically. In the sense he can learn techniques quicker than Goku can, e.g. his training on Yaardrat. The reason why Goku's better, though, is because he focuses on who he could be. Vegeta, however, preoccupies himself with who he should be. That's why he isn't as quick to discover new forms. Instead of reinventing himself, so to speak, he cares about bettering who he is currently. It's only when he sees Goku's forms that he realizes his current form isn't the strongest and seeks to better himself from there. Whereas Goku's focus is on the future, Vegeta's focus is on Goku's present. If you try to copy a dude by simply mimicking him, you'll always be waiting on his next word.