Assuming a typical 6-inch hotdog is ~50g, then a 15-foot hotdog of the same diameter is ~1500g, which is ~3.3 lbs. So that's already more total meat than the 3 lb. burger. Then you account for a 15-foot hotdog bun, which I imagine would have to have a larger total mass than a bun for the burger, particularly if the patty was made quite thick, which would reduce the needed bun surface area coverage.
So, I'd take the burger. Both would suck, of course.
Edit: And I think my 50g estimate for a typical hotdog might be quite low. (It was just the first number I found from a quick search.) Any extra weight on the hotdog just tilts things further in favor of the burger.