Around here (Minnesota) diesel costs about 5.5% more than unleaded. So the car would have to get at least 5.5% better mileage for me to even consider it.
Then you have to factor things in like scarcity. Most gas stations around here have 8-12 pumps but only 1 or 2 diesel and sometimes not even that many.
So I think I'd need about 10% better mileage before I'd consider it.