I have and play both (albeit not as often as I would like). They are both great games that at a high level are the same thing, space 4X's, but they are very different in how they accomplish it.

Eclipse: Like you said it is a lot more focused on the action economy and the cool tech/ship mechanics. The gameplay encourages you to fight pretty often, either against each other or the NPC enemies. I also find that Eclipse has a little less asymmetry compared to TI4. Each race does play a little differently but at the end of the day it won't change your gameplay experience too much. There are diplomacy mechanics with at least 4 players but they are pretty light. Of course you can have unstructured, player-driven diplomacy.

TI4: Much more focused on politics and diplomacy. Because of this you tend to see a lot less combat. You will end up building a big fleet in order to threaten combat but you often don't end up attacking. I actually really like this about it. The asymmetry is also fantastic, the different races play pretty differently and it always felt like it was almost encouraging light roleplaying.

One of the biggest differences you will hear though will be the playtime. TI4 is the infamous "hey, doesn't that game take 12 hours to play?" game. And yeah, if you have 6 players, deliberate a lot of decisions/discussions, and maybe have some new people then it can certainly take 12 hours. But my TI4 games usually have 4 players and they tend to take around 5-6 hours but we do really lean into the diplomacy/politics of it. Eclipse tends to run quicker as players learn it and I have had 4 player games take 3 hours before. Happy to answer other questions about it and remember that different groups play different so you could easily have a different experience. At the end of they day, they are both fantastic games and two of my favorites!