The down bars transfer the flex directly from the frame to the cowl. The USCT version uses the inner fender to be the conduit. I'm not convinced the inner fender won't flex some even with the USCT bracket in place. It's obviously better than no support. I think if a rigid front end is important to you and you can put up with the down bars possibly being in the way, I would do both. Belt and suspenders. I also wonder how stiff that area of the cowl is where the down bar comes in. I would be inclined to try to beef that up, but it may not be an issue depending on how hard you are beating on it. I tend to overthink issues like this, probably from being a Reliability Engineer for part of my career.
I'm putting the USCT kit on my car, and I've given a lot of thought to frame stiffening. I'd like to do down bars in front, but connected to a proper cage, but I'm not sure I want to put up with all the bars in the interior of what will be mostly a street car doing some track days.