Both are driven by systems so they're both effectively simulations, but dwarf fortress is much, much more in depth. It's basically a "real life" simulator but set in a fantasy universe, where you control a colony of dwarves. The interplay between systems is unheard of in any other game*, the closest you get is some roguelikes.
And similarly to DF it's also getting a more accessible Steam version (Space Station 14 as well as Unitystation) to make it easier to start playing it.
In my play experience SS13 wins out in terms of funny stories and variety, mainly because the speed of development is so much faster (especially on big servers like tg), it being multiplayer and there existing many spin-off forks from the usual space station gameplay.
*except maybe space station 13