I bought both. I found that the texture paste is a lot lighter and possibly fluffier when dry.
I bought some modeling paste in a tube from HL. This one seems lighter and easier to spread around than the kind in the container. It's just kind of a preference thing I think. IMO I'd start with modeling paste and see how you like it. Then you can expand from there.
Here is a discussion:
http://www.memorableseasons.com/phpBB2/viewtopic.php?f=4&t=15390
and a tutorial:
http://www.memorableseasons.com/phpBB2/viewtopic.php?f=20&t=15393
I've been playing with it using this info, very fun and cool results.