The 950 Pro is a retail drive, so it has a better warranty and support. The SM951 is intended for OEMs, so the warranty is whatever the seller offers. And it doesn't come with drivers and such in the packaging. Go with this option if the seller offers a decent warranty, you consider yourself a "techie" who doesn't need much help, and you want to save money.
The performance of the two is very, very similar.
edited to add: although I just realized, you linked to the PM951. Apparently this is the TLC version of SM951. You could make an argument that switching the NAND chips to TLC is going to make it somewhat slower, although I think supposedly Samsung has the most advanced TLC. The model you link to is, I believe, the first 3D v-NAND based TLC flash.