Fuel efficiency- Diesel naturally, especially in long trips. But in City driving, its not such a big difference in my opinion.
Smoothness- Gas engine. The diesel is smooth and refined, but the Gas wins this one.
Maintenance costs- Gas engine.
Power- Gas engine. I've seen 2.7L Fortuners sticking close to the tail of hard driven 3.0V Fortuners and 3.2L Montero Sport. The 2.5 Diesel is decently strong in the low rpm range, but I found it breathless in the 3000+ rpm. Not sure if its lack of intercooler is the reason for this.