Which one to buy? Heroes of Hammerwatch or Exanima?

Exanima appears on Steam to be in early access for about 3 years. The company is supposed to make an open world rpg backed by Kickstarter and Exanima was just a spinoff project to keep people busy until the main game arrives. In the Steam forums people are disappointed that the game hasn't been updated lately and it will probably be released by the end of the year. I wouldn't risk my money for this game.

I wouldn't buy Exanima before the actual release.
The game is in early acces since 2015.
Last Record Update: 3 months ago (January 18, 2018 – 17:34:56 UTC)

I threw the minimal pledge at Bare Mettle's Kickstarter campaign for Sui Generis (which has become Exanima now). As others have noted, it's been in dev for literally years since the Kickstarter ended, and was supposed to be targeted for release in May 2014. I wouldn't hold my breath for the game to ever be finished. It's one of only 2 Kickstarted projects I backed where I feel the money was probably wasted - but at least for this one I pledged with the attitude that I didn't really expect them to deliver on exactly what they promised (but I did at least expect some type of finished product out of it)

So yeah, I'd suggest going for Hammerwatch.

I thought Exanima and Sui Generis were different games. The company's page says

"As a result of a successful crowd funding campaign on Kickstarter we have been hard at work developing Sui Generis (SG), our ambitious open world RPG. SG has been in development for over two years and it's shaping up nicely, held to high standards and without compromise. SG itself isn't ready for release yet but as a prelude we have just launched Exanima, a standalone dungeon crawler set in its underworld. Exanima is available now on Steam or in our store."

Yes, and no. As you can see from the description you posted, Exanima is just a small piece of what they intend Sui Generis to be (intended to give players a 'taste' of what the game will offer). So I didn't mean Sui Generis had actually become Exanima. Just meant that Exanima is all we have from this company. Again, I have my doubts Sui Generis will ever actually see the light of day, it being 4 full years beyond their expected release date and they still haven't even got a beta version of the game.

Grim Dawn