This is a pretty good question.
On one hand, 5:00 is a MUCH better performance than 3 hours. 5:00 requires above average talent. A sub 3 requires average talent and lots of dedication - and, given the conditions of the contest, luck. The runner must stay healthy for most of that year, he must be healthy for the race, the race weather has to be good, he has to have a good day.
Training for the sub 5 doesn't require so much mileage. If he's a little under the weather, or, the weather isn't so great the day of the race, he can still pull it off - an 85 degree day doesn't impact the mile much at all, it would destroy any chance that any but the most talented first timers would have of breaking 3.
So, the question is, which is less likely: The chance of the guy having the talent to break 5, or the chance that enough things will go right for the average talent guy to break 3?
I think I'd go with the mile (like most on here). If he's a former athlete, and he's stayed in shape, there's a good chance we'd be starting with a guy who could at least run a 75 quarter. It wouldn't be hard to convince him he could get there in a year (even if, in fact, he couldn't)
There's also the proviso that whichever I chose, the runner would be going it alone. No way in a year *I* could hit either of those times......