Now there's always the risk that nobody wants what you made (see fendale's comment), but lets assume for a sec that that's hard either way. Here's a very dry analysis of the path to money for both:
Advertising model:
- Create something useful
- Find an audience
- Find advertisers
- Get advertisers to pay you
Product model:
- Create something useful
- Find an audience
- Get audience to pay you
That's at least one less step in the ‘product model’. You can even argue that to properly execute the advertising model you'll actually need one more step in between: “Create an ad placement strategy” (“Create” as in “design and implement” in all cases here).
Getting money from the people who experience the value of the “useful thing” sounds a lot more direct (and more efficient, which correlates to a better return) than getting the people who value the attention of the people who value the “useful thing” to pay you.
I don't have any first-hand experience with the ad-supported model, but I seriously question whether it's a/the sure-fire way of making money online.