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?
If I understood your question correctly, updating the GUI has a little to do with the way you programmed it.
From my experience, it's easier to design a main window (or whatever your top level object is) in Designer, and add some dynamically updated content in a widget(s) created in your code. In most cases, it saves your time spent on digging through QT documentation, and additionally, you are able to visually inspect positioning, aligning etc.
You don't lose anything by using a Designer, every part of the GUI can be modified in your code afterwards, if it needs some custom behavior.
Having said that, without knowing all the details of your project is hard to tell which option (QT or in-code) is faster.
QtDesigner or doing all of the Qt boilerplate by hand?