The basic LTE function of the watch is exactly the same for all carriers in the USA.
In the USA, in most markets the best coverage will vary by carrier just like your cellular coverage. The LTE watch has a much smaller antenna array compared to your iPhone.
T-Mobile offers a good deal on the Unlimited LTE Wearable plan for some cellular plans, it is $10 flat per month. The other carriers charge $10 + taxes & fees which can add up to $14 or higher.
Dave