As long as these two ‘ssk’ and ‘k1 sl1 psso’ give the same result, why not use whichever comes easier to you? 
They are both left-leaning decreases, but they have a different look. If that’s not an issue, then you can use either one. The stitch that’s been passed-over does show and is often part of the pattern. I think the ssk is a bit less noticeable.
except use sl1, k1, psso so it actually works right.
you can check out the comparison of a bunch of left slanting decrease methods that nonaknits did on it too.
:doh: That’s true, I wrote it the wrong way, of course it is ‘sl1 k1 psso’, sorry you all gave time reading, thinking and writing this. Well, yes what I proposed was “Doesn’t ‘ssk’ and ‘sl1 k1 psso’ look the same in the end?” GOOD KNITTING
No they don’t look the same in the end.
The SKP (Slip 1 K1 Psso) has a leggier left slant, usually prescribe by the designer to give “a look” for a leaf or a leafy appearance.
The SSK just simply decreases a stitch with a minimal left slant. In which case I use the SSK “improved”…as shown on this website under basic techniques/decreasing.
On that plain ole swatch it doesn’t seem to matter, but in reality, it matters.
I always use the style of decrease that the designer prescribes. “If it ain’t broke, why fix it?” is my motto. She is the designer, I am the knitter. That is what I pay her for…her designs! 
I think they do, at least when I knit, they look the same. I’ll use both on the same piece depending on how my brain is functioning, or what seems to be easiest at the time.
And I don’t think the `improved’ SSK where you sl the 2nd st pw looks good; it twists that 2nd stitch and doesn’t lie flat.
sue