Apple Pay will still work on GPS only watch, even if you don't have your iPhone with you. Mine is a SS version that has cellular but I didn't use. I sync my Apple Music playlist to my Watch and I run without my iPhone.reporting here......
Anyway, I love cellular as I like to jog or walk with my watch alone which is me time without disturbance from iphone.
Then, I can buy something on my way using Apple pay on my watch.
But really depends on usage, likely I will get non cellular version for my wife.