Absolutely, you need both. I'm not sure how to quantify which one is more powerful or important, especially since "idea" can mean many things, from a very vague concept to a fully fleshed out and vetted business plan. I was just taking issue with the "ideas are worth nothing, execution is everything" mantra.
Let me quote Derek Sivers: "To me, ideas are worth nothing unless executed. They are just a multiplier. Execution is worth millions."
That should go without saying though. I think a better way of asking the question is "what's better: a brilliant idea executed so-so or a so-so idea executed brilliantly?"
That to me is a more interesting and logical question.
A fully fleshed out and vetted business plan has long left the idea building...