Evidence for both exist in the wild. 'Build it and they will come' seems the more elusive, so in a generalist sense I would go with the 50% marketing. That said I'm a marketer so what can you expect!

In my sphere I've seen so many companies over focus on the product and be dismissive of marketing when It's clear they can grow there business cost effectively via advertising.

I've also seen business throw money down the drain trying to buy their way to success and unreasonably overcome the product weakness.

IMO the key is measurement of success/failure with marketing. And balance in product/marketing focus. I read a great book that covers some of this call 'In Search of Stupidity: Over 20 Years of High-Tech Marketing Disasters'.