About $60 difference money wise.
Pro4000 resolution for still image goes to 1280x960 at 1.3 megapixels (logitech does not provide lower resolution figure for entry web cam.
video frame rate of 30 fps is at lower 320x240 for web cam.
brochure links:
http://www.kis-kiosk.com/pdfs/logitech-express.pdf
http://www.kis-kiosk.com/pdfs/logitech-pro4000.pdf