TL;DR: in line with the consensus in the community, XCOM2 is a better game than XCOM Enemy Unknown and Within, though the extent to which it is is somewhat exaggerated.
I have played XCOM Enemy Unknown (released 2012) since 2012, and have done about a dozen playthroughs of that game and about 2 playthroughs of the Enemy Within expansion. I’ve only known XCOM 2 (released 2016) for about 2 years and finished it twice, so my take on it isn’t as thought out.
In these games, you control about 4-6 units that move around the map, hide behind cover, and shoot at aliens when they find them. Sounds rather dull, but the experience itself is much better in practice, due to the addition of classes, items, types of aliens, and whatnot. Easy to play with friends, with a podcast on, or in tryhard autist mode.
Besides the main game, there is a side game of base management, where you manage the resources, buildings, research, planes, and satelleites of the organization; arguably two games in one. Throughout the game, the enemies scale up in terms of power, so you will need to upgrade your soldiers and research technology in order to compete with them.
To some extent, the goals of both minigames reinforce each other: you play missions to get resources and staff, you use that capital to build up your research, facilities, and military, and then you use that infrastructure to win on the field. It sounds circular, but it works well in practice.
The game is highly customizable. In Enemy Within, you can name your medals.
You can name soldiers in both games.
The main difference between XCOM2 and XCOM Enemy Unknown/Within is that XCOM2 is more refined than its predecessor. The graphics are better, the UI is streamlined, there are more women and minorities, and so on. I don’t think this is necessarily a bad thing, if anything it puts it above the original.
Where the second game went wrong is that it was too cluttered. There’s a lot of different types of ammo, stealth mechanics, items, types of enemies, hacking, rewards, and whatnot that make it a more scatterbrained experience. By virtue of this, it also has many more bugs.
Gamers for whatever reason are convinced that more content is better. It is not. This is partially true for RPGs like Skyrim or Oblivion where more weapons, spells, armor etc make the game novel and interesting, but for strategy games, too many mechanics can make the experience more draining.
The second game has the pretentions of the story, but is largely dystopian boilerplate, a genre that was already overwritten 50 years ago. The goals of the aliens and the lore behind it are interesting, but it feels a bit too Star Wars for my taste. The first is a more blatantly a storyless game, which I consider to be more respectable.
XCOM2 fixed the pacing problem, where the original XCOM encouraged overwatch crawling in basically every mission; the concealment mechanic and the timer result in that being the optimal strategy less often.
In terms of atmosphere, the first game is better and has a nice, unnerving atmosphere. XCOM2 is too modernized and generic.
Base management in the second game is less free and easier, in the first it is a skill in its own right.
The sound effects in both games are great, but the ones in the first are better.
In all games, the final mission is great.
In terms of Unknown vs Within, Within is better just by virtue of having human vs human combat and the base defense mission. Most of the other additions are good, but they build up the cluttering problem that is observed in the next game. The difference between the games is small, so minimal that they may as well be considered different patches of the same game.
Notes on gameplay:
XCOM2: Serial, Sniper/Ranger synergy, free reloads, serial/faceoff synergy (in different characters) — all broken.
Both games get easier as you progress through the game, though this is especially true for XCOM2.
In XCOM1: snipers are OP late and heavies are OP early. Assaults are balanced and supports are mid.
XCOM1 optimal build order in early game is weapon fragments/satellite/excavate →first mission (choose engineers)→research beam weapons →build uplink ASAP. This is a good guide for all modes.
XCOM1 assault: take lightning reflexes, rapid fire, close combat, and resil. The crit build could be used on a second assault, but LR is too good to pass up.
XCOM1 sniper: squadsight is not as good as people say it is — snapshot sniper is perfectly doable, especially late game where aim boosts are easy to come by in items or abilities. It’s straight up better than squadsight in Enemy Within, since the aim penalty is minimal. Take ITZ (if snap shot), or double tap (if squadsight). Pistol builds are good, but imo it’s too much of a drag to pull them out.
XCOM1 heavy: take bullet swarm, shredder rocket, HEAT, danger zone, and rocketeer.
XCOM1 support: go medic.
XCOM2 ranger: the sword is a gimmick. Go for a flanker/concealed build.
XCOM2 sniper: the pistol build is better early, but the regular sniper build scales better.
XCOM2 heavy: both grenadier and gunner builds are viable.
XCOM2 specialist: both the hacker and the medic are useful.
XCOM2 last mission: in terms of utility, rangers > specialists > heavies > snipers. Bring mech piercing rounds on at least one person. If you have a pistol sniper and a squadsight sniper, bring the pistol sniper. Bring at least one of every class except for sniper, where it is optional. Just focus on killing the avatars — the aliens are largely a distraction.
XCOM1 last mission: snipers > heavies > assaults > medics, just like in the regular game, though I would recommend bringing at least one of each class.
Don’t start the game playing ironman. But ironman is funner if you are a veteran.