Apple Watch

If you are going to use it for exercise without carrying your iPhone, and you have a cell provider that supports the Watch, it is nice to be able to use GPS with maps and be able to stream music to the Watch. I use the path at a nearby park and find it really nice to leave the bulky phone at home. I'm also way to prone to forgetting the iPhone and I'm able to keep in contact without it.

A GPS only model can do "EVERYTHING" the cellular model can do as long as your iPhone is nearby.

The following web page will help clarify the differences when the iPhone is not nearby

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

Make sure you read the footnotes.

I got the GPS only and as I always have my iPhone with me, or somewhere in the house (Bluetooth range is rather good), I am very happy with the GPS only model.