I find SD to be very similar on me to the grey from the Pret-a-Papier collection. It's very flattering and is good for day or night. I do have darker skin around my eyes, and I think that helps it to look nice. If you're very pale toned around your eyes, it might be a little bit on the dark side for daytime use.
It's Phys is really versatile, because you can use it to line, it looks great smudged under the eye, plus it's gorgeous all over the lid in a thin layer too, or used as a base. It's cool toned, especially compared to SD, so if you look better in cool tones than warm tones, you can factor that in. The pink shimmering highlight it has is just so beautiful. I backed it up and I almost never BU fluidlines!!!!
I guess other questions you can ask yourself is how many grey eyeshadows you have compared to how many grey liners you have, plus what is your eye color. If you have green eyes, IP will enhance them more, blue eyes it's possible SD will be more enhancing. Brown eyes could go either way and you do have to take into account if you can wear cool tones around the eyes without them making you look tired.
I assume you can't see these in person first? Good luck on your choice!