There is a difference in the way a Softar renders the light compared to a "regular" soft focus filter. I can't describe it but if you can find images online it should help.
@ $30 ea, for the Softars. I'd most likely go with them. But since you already have the Lee system that makes a strong argument for Lees.
If you do portraits with both the Nikon and Hasselblad, it should be possible to get adapters to use the Softars on the Nikon.