Terraforming Mars is in my top-3 solo games at the moment. I'm also closing in on 100 plays, which is not the same, as I play some favourites just a handful times a year. It's often compared to Underwater Cities, which happens to be my most recent acquisition. And while I love Underwater Cities as well (I've played it 12 times, and the first week isn't over yet), something weird happened to me. The game never, ever, made me think of Terraforming Mars.
Of course you can compare Underwater Cities to Terraforming Mars. When we are talking component quality for instance. Good lord. And yes, both have cards and a tableau. But I'm more interested in gameplay. Let me try to explain in broad strokes (ignoring some cool twists both games have that make them each great in their own league).
Underwater Cities plays like your typical euro worker placement game. Like in Nusfjord for example, every turn you place one of your three workers to get or use resources - and these workers block your opponent (or in solo: the AI blocks some actions from you every turn). With your hand of cards and the tableau you try to get temporary bonuses that will help you during gameplay. There are a few separate production rounds (that feel a lot like the harvest phases in Agricola, you even have to feed your workers 😨) and the usual over-the-top euro end scoring round. In this round everything - from cities to buildings to some cards to resources - is converted into victory points. VP kelp salad.
Terraforming Mars is an engine builder, where you produce resources every round. Then use them to produce more resources in your next round etc. There are no workers to be placed, no action spaces to be blocked. The amount and availability of your actions are only limited by the resources you have. You lay out your cards in a tableau, and the first game that comes to mind to compare it to is Race for the Galaxy. It is no coincidence that the card game version Terraforming Mars: Ares Expedition will have similar action phases as that game (or as Puerto Rico if you want to be pedantic).
So. If you enjoy playing similar feeling games:
For Underwater Cities, try one of Uwe Rosenberg's euro games. These are the Everdell killers.
For Terraforming Mars, try one of Tom Lehmann "for the Galaxy" games. These are the Wingspan killers.
If you like variety in your collection like I do, Underwater Cities will coexist happily with Terraforming Mars. Go for the fruitbowl! Add Elder Sign. And Comanchería. And Legendary Encounters: Alien. Live and let live, I say. Or you'll get lonely.
So you live in the bottom of the sea,
And you kill all that come near you...
But you are very lonely, because all the other fish fear you