I think I'm right in saying that diesel engines do use less kg of fuel per hour for a given output - but they also do weigh more.
If you call the weight of the engine (EW) + fuel weight (FW), and use the prefix D for diesel and P for petrol, then for a short trip, (DEW+DFW)>(PEW+PFW), and for a long trip (DEW+DFW)<(PWE+PFW).
The breakeven journey time obviously varies with the plane/engine combo.
So perhaps it's more a question of what type of flying do you do which determines the best choice for you....
As for reliability, the Rotax 912 and variants are supremely reliable - and with the huge numbers in use out there, the development curve is solid. As of today, diesels are in far less common usage, and no doubt they do theoretically have the potential to be more reliable, but I'm not aware of any data that proves that today's available engines actually are - they may be less reliable.