Starmade and Space Engineers are about as similar as Roblox and Blockland are to one another.
I don't own Space Engineers, but I've seen gameplay videos of it and I want to buy it when I have the money for it. Starmade- I've owned it for a little over a year now and it's very fun to play.
Resource-intensive-wise, you can build more objects in Starmade than you could in Space Engineers without as much of an impact on performance. The difference? Starmade's block system is a lot like Minecraft's and so everything is pixelated to a degree and is much simpler. It isn't as stable bug-wise as compared to Space Engineers, but IMO there is still much more to do in it.
Space Engineers has ships built in blocks a bit like Starmade, but they are much different- Each ship block in Space Engineers is much more detailed than the same Starmade ship block, and they're also bigger too. They have detailed models for each block and so it's kind of like a combination of, well, I don't know, maybe Moonbase Alpha with Minecraft and Freelancer.
Starmade's ships are considerably more customizeable than Space Engineer's ships are, and have more shape relative to their size. In Starmade, you can use hundreds of different blocks to decorate your ship. Since the blocks are minecraft-sized, a decent starship hallway can be about 3 blocks tall in space, with a 1-block-thick ceiling and floor. In Space Engineers, your hallways are typically just 1 block high of 1 specific corridor block, with some specialized door blocks and whatnot.
Personally, I don't see any reason why you shouldn't just go ahead and get Starmade, because it's free at the moment. When you have the money for Space Engineers, get that too. They are both lots of fun.
One thing to keep in mind is that Starmade is a game being developed by only one guy who is new to the Indie video game industry. Space Engineers hasn't been around as long but it's being developed at a much faster pace because it's got a full team of professional game developers making it. I suppose therefore that's why it's more expensive.
StarMade will continue to be free for probably several more months until the game reaches Beta. You can just pay $3 or so to "register" your account- Paying for it doesn't change anything except make your account a registered one. There is no demo version; You just go to the website and download the full game for free.
Source: I am a member of Starmade's Dev Tester staff
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