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?
First of all, the requirements that you've mentioned don't (or shouldn't) have much affect on this decision.
Either way, you're going to have to learn something.  You might as well investigate both options, and make the decision yourself.  Write a couple of "Hello, World!" apps, then start adding some extra widgets/behavior to see how each approach scales.
Since you asked, I would probably use Qt Designer.  But I'm not you, and I'm not working on (nor do I know much of anything about) your project.
QtDesigner or doing all of the Qt boilerplate by hand?