It depends entirely on who you're trying to sell to. If you're selling to businesses, then it's an easier job getting them to pay you directly.
Personally though in my experience of consumer based websites, making money from advertising is 'easy'. Orders of magnitude easier than getting someone to reach for their wallet.
Lets say you create some webapp that you charge $20/mo for. To get to $50M a year, you need to get 200,000 paying customers.
The question then is what is harder to achieve, 200k paying customers, or 4bn page views/month.