If they aren't scratched, the used softars are just as good as new ones. Probably "better" than the Lee equivalent, especially at 4x+ the price.
Though for a soft focus attachment, I'm not sure how you would judge which is better optically. It would really come down to which one gives you the most pleasing fuzzy result.

As for other types of filters, I'd be surprised if the Lees are better, but they may not be that much worse, if they were, they wouldn't sell so well.

Since you already have an investment in the Lee system you're probably better off to continue working with it, and you will easily be able to switch them between systems. If you need a particular filter, like the softar, that you can get at a good price buy that if it's something you can easily dedicate to the Hasselblad.

The Softar wouldn't preclude you from adding a Lee in front of it, if you needed to, though there would be some risk of vignetting.