When starting up a new project, as a beginner, which would you use?  
For example, in my situation.  I'm going to have a program running on an infinite loop, constantly updating values.  I need these values to be represented as a bar graph as they're updating.  At the same time, the GUI has to be responsive to user feedback as there will be some QObjects that will be used to updated parameters within that infinite loop.  So these need to be on separate threads, if I'm not mistaken.  Which choice would give the most/least hassle?
Your right threading is your answer.  Use the QT threads they work very well.  
Where I work when people start out using QT a lot of them start with designer but eventually end up hand coding it.  I think you will end up hand coding it but if you are someone who really likes GUIs you may want to start with Designer.  I know that isn't a definitive answer but it really depends.
QtDesigner or doing all of the Qt boilerplate by hand?