Hubble could not observe the mid-infrared region as its mirrors were not cool enough, meaning that infrared radiation from the mirrors themselves would have dominated any attempted observations.
The Hubble image shows dark regions that seem to separate the spiral arms, whereas the Webb image shows more of a continual tangle of structures that echo the spiral arms’ shape.
Those same dusty regions are no longer dark in the Webb image, however, as the mid-infrared light from the galaxy has been able to pass through them.