I have used both. The pre-packaged Carib Sea is great, but it only carries the biofilter in the sand, not "critters".
The Keys live sand is awesome. Its been several years but I loved this sand for its particle size variety. I used it to set up my 72 gal reef which was totally filtered by 5" deep sand bed and live rock (also from SWF). This sand was completely cycled. I had zero ammonia, nitrite and natrates from day 1 and never had any thing above zero for the duration of having that tank. Now, I was living in the Keys at the time, so there was not any delivery time delay. I got the sand and rock together. Smelled great (like the ocean) when I opened up my payload of stuff. I also used some more down the line just to replenish the DSB.