Yes, it is definitely worth it for the HDMI capability. As far as the extra HD space, it's probably a moot issue right now, but if you're intending on DL'ing a lot of the online movies, they'll definitely add up.
There is talk of the new units (once they lower the prices) having the premium system still with the 20GB HD but with HDMI.
This is probably true, because they are going to release a "Halo 3" edition 360, which will have a 20GB HD that includes HDMI access.
You can transfer saved data via USB - although if you've bought anything from the XBL arcade, you don't have to transfer that if you don't want to, it's tied in with your gamertag, you'll just have to re-download it.