The ones for aquariums are prolly soaked to remove the acids that turn the water brown and could be a different kind that sinks. If reptile stuff is cheaper just buy that and soak it in a bucket first to get rid of the stuff that would otherwise leech out into your tank, and so it will sink.
The driftwood I bought from Petco.com says it's ok for aquariums. Here's the link.
The thing is that it's pretty rough in some spots, not polished down like a lot of driftwood. I'm soaking it for a good week or more, but does the texture make a difference? I don't want a fish massacre