When you use data-structures (strings, lists, sets) in your qt project code, do you use the std ones, or the qt ones?

How do they compare size/speed wise? And when, for example, I want to use an std::string in some existing widgets, will they need be converted to a QString (automatically or otherwise) first?

I'm trying to make this decision. I'm probably going to use std classes, though, since I'd like to make a command-line version of the same application, and I'd like to avoid using any QT libraries in that.