I am over here to tell you the difference in terms of a user's point of view.
Difference as a user:
I have used both the devices for months each. Main difference between the two is offcourse the network connectivity. The LTE variant uses an esim so you can use the same number as your phone(depends on Carrier, Airtel Postpaid and Jio have it) whereas the bluetooth variant needs a phone to be connected. In terms of usage the main difference is that the LTE variant can connect to your phone remotely. So when you are not near your phone then too you can get calls and notifications. Best use is the use of Spotify app where in the LTE variant you can even stream music. Bluetooth version can play only music taken offline. There is internal memory in both of them Battery strength of the LTE variant is offcourse less as it needs to search for the network and connect constantly. In the LTE variant if someone calls you then you can take the call using its own speakers or you can connect a pair of bluetooth earphones/headphones of your choice to take the call. In the Bluetooth version you can only take the call using the watch speaker and not using a blutooth device. This might sound wears but you need to have the earphones connected to your phone to take call via it. Other uses remains the same. I use the 46 mm variant. I love the extra battery life it provides.
I prefer my galaxy Buds which go great with my Galaxy watch 4G. You too can get this combo.
Spotify on Galaxy watch Blutooth
Spotify premium on Galaxy watch LTE
I simply love the flexibility of the LTE version where you can stay away from your phone. What is your Views? Do comment and tell.