TBTs, which is great for me as I don't use them for anything else (I have no idea what 'collectibles' are for, nor why I'd want any) and it saves me having to spend any in-game resources to get the things I want.
It's a win-win, really, as the people I trade with clearly value the TBTs (although I suppose I do too as a resource to trade with).
Collectables are pretty little pictures I guess people use to express themselves more. I'm on the same boat as you though I use TBT for trading only but I could see myself getting into collectables in the future