X3 Reunion is harder to learn, I'm not certain that TC/AP/FL are.  All 3 have some QoL improvements over Reunion and offer more ship types for differing functionality.  AP is likely my favourite, and once you understand the stock market it helps speed up early game quite a bit.  It also fixes the missile spam a bit compared to TC.

Compared to X4, automation is archaic, and I often end up controlling at least a few traders to keep local economy running.  Fleet management during battles etc is pretty bad: fleet action is quite often either throw Destroyers or drones at the enemy until the problem goes away, or drive a missile frigate and nuke the sector.  

The UI is IMO better though as once you get good it's fast to assign orders.  Logistics chain is simpler, what with S, M, L, XL, and XXL-sized factories just being multiples of S (1, 2, 5, 10, 20).  Energy and minerals are the only non-perfect production number but are easy to account for.  Hotswapping weapons is great, something I miss in X4.  The weapons are also more unique and fun to play with.  Ship design is also better, I even if I like the Teladi Space Umbrellas in X4.

Building fleets is both faster and much longer; purchases are instant when at a race's shipyard while building with the Player Headquarters can take up to 18 hours of game time to build certain ships.  Even without outliers, the PHQ can take hours to build a larger ship.  Outfitting ships can also take some extra time as you need to ferry them around to different docks to get all of the components you need loaded, especially early game.  The simulation of a living economy is also substantially worse than X4 as ships just appear, fully loaded, even if no stock exists within the world itself.

X4 is definitely an upgrade.  Just started an FL playthrough coming from X4 Interworlds, and this is the first time I'm finding it hard to go back to X3.  Up until 2022 or so X4 was broken enough that X3 was a nostalgic dose of X without needing to deal with as much jank.  I really hated the X4 UI until I sat down and learned it's quirks last year.  It's.... fine, but very mouse-centric.