Once you've seen the calcium carbonate sand the difference is obvious. silica sand is 'sharp edged' and has the a crystal or diamond like appearence. You'll notice it reflect light as you roll it around in your hand. The calcium based sand I think comes from finely crushed seashells... just a guess though. But when you look at it it 'looks' softer that silica sand. It has a chalky feel to it.
"Silicate free" means nothing. "Silicates" are toxic to humans and there used to be a problem with them like there was with asbestose. You'd inhale them and over time get sick. So people that sell sand like to put that on the lable so you know what you're getting isn't toxic.
I've read several articals about it written by marine biologists and they all said that there was little to no benifit in using Calcium carbonate sand. But they admitted that the small crustaitions living in the sand may benifit due to the lack of sharp edges in the calcium based sand. There is a myth that the silica sand leaches silicate into your tank water. But the glass your tank is made out of is nearly 100% silica... if leaching were a problem the sand would be the least of your worries.
I use calcium based because it's a hell of a lot cooler looking than regular sand.
I've also read an artical that said that you can colonise Dead sand with live rocks, but you wouldn't get the bio-diversity that seeding it with true live sand would. Basically, there are some animals that live in Live sand, some that live in live rock, and some that live in both sand and rock. If you seed the sand with live rock you'll only have the kind that live in both live rock and live sand. The creatures that only live in the sand wouldn't come from the rock. But I only read that in 1 artical and haven't seen it confirmed elsewhere.
Personally, I got some of that "Agra-alive" stuff and live rock. Despite the agra-alive sand being questionable, it worked well. I have so many little critters running around its nuts. The sand practically undulates with life.
By the way, if you bring a cute girl over to your house, don't point this out to her. Trust me, cute girls do not appretiate bio-diversity and all the cutesy baybe talk she was directing twords "little Nemo" soon vanishes. I learned the hard way. (I really should call her...)