The difference in price between Plus and Ultimate is so little, you may as well go for the Ultimate. The three add-ons, are something I haven't really used too much, but are nice to have.

Soundsoap - cleans audio (this program sells for at least $99 separately)
Vitascene by Prodad - a scaled down version of the full program that sells for a couple hundred dollars I believe on Prodad's site - this has some nice transitions and some filter effects that are keyframeable. You need a good video card to use it.
Moving Picture - This used to be a $199 plugin, now included. It allows some rotation of pictures, etc., previously not available in Studio.

Some people reported issues registering Soundsoap - I had no problem. Vitascene didn't work well for me til I upgraded my video card (something I needed to do anyway).