>>2049322There's three actually: Aftermath, Aftershock and Afterlight.
As the other Anon said Aftermath is a bit simpler that's not necessarily a bad thing because Aftershock can get a bit tedious sometimes. From what I've read online most people prefer this one.
I think I'm in the minority, but I actually like Aftershock the most, simply because it has more options to play with including psionics and cyborgs. It starts off from one of the worst endings from Aftermath (allowing the aliens to relocate your dudes to a space station while the rest of the Earth gets fucked) which helps explain why it wasn't as well received.
Afterlight is a bit of an odd one, not just because of the cartoony style but also because you play with a fixed cast. You can recruit a few characters along the way and there are some characters that only show up as replacements if you let anyone die, but otherwise the crew you start with is what you have for most of the game. IIRC this one takes place simultaneously with Aftershock, dealing with a research station trying to terraform Mars in order to create a new home after the Earth got shit on in the canon Aftermath ending.