Don:
The 355N was a more refined model over the 355F model both in cruise, power and engine controls fadec). Bigger payload too.
Comparing the 355N to a 350 -
The 355N is a heavy aircraft for its class and pushing out around 520shp each engine with an empty/full weight of 1490kg/2600Kg. Payload: 1100kg. A twin engined, FADEC with associated expensive running costs but it did have the 350B3 gearbox.
The 350B3 is pushing out 850shp with an empty and full wt of 1100kg/2200kg. Payload: 1100kg.
Single engine with associated cheaper running costs and all the world records!
So why go for the twin (unless in the EU over water/IFR etc.)