Personally, I believe that both are dead civilizations existing in a history that matters only because of the value we put on it.
"During the period from 1519 to 1848, all or parts of Texas were claimed by six countries: France, Spain, Mexico, the Republic of Texas, the United States of America—as well as the Confederate States of America in 1861–65."
Texas has independent history of something like few decades, and while term "civilisation" is unclear, I am absolutely clear this lone state doesn't suit into any definitions. It was not empire, not long - lasting sovereign state, its cultural impact is at least questionable, its impact on world history was no bigger than impact of any US state alone.
Saying this is exactly the same as Roman Empire, which direct or indirect impact on world history has ~2 500 years, and which basically "created" Europe, which created Spain AND which created Texas is laughable. While it is definitely impossible to say "this culture is BETTER than this", saying it is impossible to measure power/impact of political/ethnic groups basically denies whole history, historiosophy and concept of civilisation. No, Rome matters because it had extremely big impact on culture, political history, architecture, militaries, economy, while Texas DIDN'T HAVE IT.
Saying "Rome is considered more important than Texas only because of value we put on it" is pure logical absurd, it is like saying "1000 dollars is more than 10 dollars only because of value we put on it". Uhm, yes, exactly, so? Maybe I am unclear but to be honest I am astonished by concept of water being more wet than sand only because we put value on it
Sorry, Rome was simply more important than Texas and it is regarded as more interesting. For most of customers and creators of Civ series Roman Empire is waaaaay prefered over Texas State, so I wouldn't expected it unless developers start making Civ XIV with 150 civilizations, including Texas, Masovia, Northumbria, Xibei Sai Ma, Vladivostok, Neanderthals, Hessen, West Steppe Under Control Of This Unnamed Horse Tribe, and so on.
My own 9 favourite civs to appear in the series:
- Great Zimbabwe/Monomatapa/Mutapa
- Kilwa
- Ghana/Hausa/again Mali
- Cahokia
- Hungary
- Tartars
- Vietnam
- Khmer again after CivIV
- Israel