I've played both XCOM:EU/EW And UFO:AL for a significant ammount of time now, but I like Afterlight better overall.
The depth of the game easily rivals if not outright defeats XCOM. The weapons can be modified with various attachments and ammunitions, and there are dozens of weapons, most of which are still useful by mid-lategame. Equipment loadouts range from weapons to inventory including spare magazines and more weapons or tools.
It also does not burden you with pre-selected classes and equipment restrictions [Which cost me quite a few XCOM:EW playthrough attempts] since all your crew are capable of the same training and skills [Though alien teamates have different training from humans], which are also more indepth than XCOM. You can train military personell to run faster, damage specific species better, use heavy weapons/suits, or handle weapons faster to name a few categories.
In XCOM's strategic play, you have to manage your money, alien resources, and research projects while scanning for alien ships. In Afterlight, you also have to attack and seize territory, and decide who to be friends with. Will you create lasting friendships or do eye their territories, looking for more land and resources?
The game also has lasting consequences and effects. Team members aren't generic soldiers who happen to be a notch above the rest of millions. You've got a colony and a couple-dozen people to staff it, and those who die are gone, they aren't cloned or replaced, just dead.
On that topic, terraforming [Because thats an obvious goal, right?] has visible overtime effects that ever-slowly escalate visibly on both tactical and strategic play. As terraforming carrys on over the days, you'll see the world turn green with blue slowly, and tactical maps will gain plants and grass graphics too. On top of that, more overgrown areas tend to have lower environmental hostility ratings for tactical battles.
The biggest upside it has, Afterlight's action is real-time with pausing and planning rather than XCOM's turn-based action. Running head-on into previously undetected enemies or missing a shot is no longer a death sentance for your troops, though avoiding the former is still advised. Realtime action and positioning all play a role, where a sniper lying prone on a cliffside can spot enemies and clear them out quickly, and cover is less important and less of an invasive mechanic [though when you get the chance, use it]. Stealth is also an option, with smaller suits, stealthy movement training and silencers all present and accounted for. In a rescue mission, I snuck into the camp, and extracted my target without having to kill more than 2 or 3 enemies patrolling that area.
Although Afterlight is in many respects more realistic than XCOM, it still has downsides. The graphics aren't up-to-date and the dialogue switches tones and word-accents [You'll see what I mean if you watch videos with story dialogue.] The most invasive bugs such as the "Green Grid Error" as I call it where most of the map in tactical battles covered with green grid squares. [There is no explanation at all for these grids. Could they have something to do with graphical errors related to terraforming? I think it's an unknown error with my settings but who knows?]