Well I was recently playing the demo myself, but got the full version last week and Entrenchment a few days later, so I can answer this question.
- Patches: The demo doesn't have any of the patches, meaning that several things change. Like in the demo, carriers only carry 1 squad each and take some 8 supply points or something. In the most recent patch they carry 2 or 3 squads each, cost significantly more, and take up more SP. Some bugs are also fixed
- The other races: They help add some variety to the game. Like I personally play the Advent, which I obviously couldn't do in the demo.
- Maps: The huge maps are awesome. I usually play on random, so I like being able to have bigger maps on that setting. Over 100 planets in five different stars.
- Time: It is nice being able to play for more than 90 minutes.
- Multiplayer: It is also nice to play with other people, and since Sims doesn't have license limits, you can put it on all of the computers in your house and play with a lot of people even if you don't like playing on internet (Pretty much everyone there spams units btw, so it isn't that fun).
As for Entrenchment, you're basically paying $10 to not need a fleet to defend a planet. I got it, it's fun, I'm glad I did, but that's what it boils down to. You'll also need Entrenchment to play online, as that's the online standard right now. I couldn't tell you about diplomacy, but it's not feature frozen yet (from what I heard), so no one will be able to tell you about it with 100% accuracy.
And so yeah, I'd say that it's worth at least buying vanilla sins.I would advise not getting Entrenchment for a few days after you get vanilla and see if you think you need it.
