So, how big difference there is between these editions. Is it worth using extra bucks on the ultimate edition.

Can I open/edit studio plus 11 project file in ultimate edition and vice versa?

Studio 11+ and Ultimate are EXACTLY the same program!!! Ultimate has three extra sub programs added. These cover Sound cleaning (BIAS SoundSoap), animation of still shots (StageTools Moving Picture) and extra effects on video.(ProDad VitaScene)
No problem as it is the same basic program.

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).