I can say Stag makes a great product no matter what they push out. I have a friend with one of these and he enjoy's it - albeit I cannot comment about other brands as I have no experience with them.
Does Stag actually make the kit or does someone else make it for Stag?
I have experience with the CMMG kit, which is similar to the Ceiner and Spike's kits, and would recommend it to anyone looking for a cheap way to plink with their 5.56 gun.
I have a complete STAG, too. I'm aware STAG is a company that makes all of the parts for COLT, DPMS and others - they decided to make their own brand. From what I've read Stag makes one of the best conversion kits - they work best in STAG receivers of course.
The pictures would indicate that it is its own kit, as the CMMG, Ceiner, and Spike's kits have small design differences. Those are CMMG magazines, however.