The demo is just an older version and without multi player support. In this case, it's from March, before Block 2 was added. Check the changelogs in the update posts to see all the differences and additions that the latest Early Access version has.
There have been some minor GPU memory improvements since this demo version, but overall you should get a pretty good impression of how the game runs on your system before you buy it.
Remember: "buying" here means most and foremost supporting the development of the game or just because you like it and had some fun with it in the past. You also get a little bit in return with a couple of new features and multi player support. But it's not that much and there is no reason for me to have sales. So instead of waiting for a sale, get the demo and if you feel like giving that extra support and getting those extra features, get the EA version any time.
Thank you! :)
