I am looking to get a watch for an iPhone X

The GPS only Apple Watch can do  Everything  the cellular model can do  as long as  your iPhone is  nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).

Yes the GPS only model can do SMS/MMS texting as well as Internet based messaging, such as Apple's iMessage. The GPS Apple Watch just uses the iPhone to do it, but you can send and receive the messages on your wrist. My wife and I have GPS only Series 4 models and use iMessage all the time, even when we are both at home.

Actually you need the iPhone to receive SMS/MMS texts regardless of whether you have a GPS only or Cellular model

https://support.apple.com/en-us/HT205547

Read the footnotes

With the Cellular your iPhone does not need to be with you, but it must be power on and able to talk to the Internet so it can relay SMS/MMS texts to your Apple Watch.

Internet based messaging, such as Apple's iMessage, does not require the iPhone as a relay for the Cellular Apple Watch.

The Cellular Apple Watch shares the iPhone's phone number, so the iPhone cellular carrier must support the Apple Watch. For this phone number sharing service, the iPhone cellular carrier charges a monthly fee. In the U.S. the fee is $10 per month added to your iPhone bill.

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

https://www.apple.com/watch/

The one with cellular can work to receive/send calls, messages, etc. even if your iPhone is not with you. For the GPS only model to do these things your iPhone must be with you. Keep in mind only certain carriers are compatible with the GPS+Cellular Watch and you will pay a monthly fee of around $10.