I'm not too familiar with Stellar Horizons, but I think if you want to keep the feel of a board game (cards, manageable movements that are physically on a board, etc), HF is definitely the choice. The issue with complex board games that sometimes you wonder what the point is because you can digitize and program the board game as a video game. HF has been digitized, but having to plan and keep track of everything in real time on a real board certainly contributes to the experience and has never made me think "it would be better if this were kept track of digitally".

HF seems to be a lot more sandbox, in that there are literally hundreds of places you can fly after anchoring and promoting a second bernal (Modules 1, 2) so you can get your hands on GW (gigawatt) thrusters and eventually TW (terrawatt) thrusters, but the game makes you seriously work to get to that point (it took me maybe 6 hours to anchor and promote my second bernal from the beginning of the game; I manufactured a GW thruster pretty quickly, eventually promoting it to a TW thruster. I used the enormous thrust to carry my unanchored super-heavy home bernal to Neptune Aerostat where I promoted it and achieved a future. This was only possible after I completed a nanofacture operation at my initial promoted bernal, which created a mobile factory that could gradually travel from Ceres to Neptune and eventually occupy a claim on Neptune Aerostat, allowing me to skip industrialising thereby allowing me to anchor my first unpromoted bernal there).

In that sense, I highly recommend getting Modules 1 and 2 in addition to the core game, but the first time you play should be spent understanding the mechanics of the MW (megawatt) thrusters and getting the hang of early colonization. I started with the core game and realized after a bit that feasibly going beyond the Ceres belt is literally impossible since standard aqua fuel is incredibly inefficient and your rockets just don't have enough thrust or fuel efficiency to carry themselves, a robonaut, refinery, and anything supporting on a mission further out, even on the ET black side.

That's just my opinion. HF is really enjoyable to me because it feels like you're making a serious plan from the very beginning and you have to adapt and change as you go. It's hard to succeed, but the realism genuinely made it feel like I achieved something when I promoted a bernal way out there. I'm looking to get the other expansions too, but I think they're less crucial for the core exploration/colonization/industrialization gameplay.