"Softness" geneerally means "not sharp".
When one use a soft filter, the entire picture is made a little "hazy" with less harsh lines, which ladies may prefer, if I am allow to make cliches statements. However such softness may not be desirable when photography fishermen with all the "wonderful" lines.
Regarding "softness" in lenses used wide open - the image should be sharp where it should be, and out of focus when it is desired to be soft or out of focus. For example, in using a 80mm f1.4 lens wide open, one should be able to count all the eyelashes, yet the ears out of focus, giving an illusion of depth in a two-demsional image. This image is technically not "soft", but parts are rendered out of focus for esthetic reasons.
When a lens is called "soft" we may be talking about a poor quality lens which do not focus well, resulting in nothing in focus.
Coming back to the original question - what filters for portrait. The question I will ask is this. Are you making black & white portraits, or colored. Film or digital. Without more information, any answers will not be properly directed.