Here are the differences between Apple Watch Cellular and GPS-only models
(iMore iPhone)

Tethered Apple Watch GPS From $199 at Apple Pros Less expensive Store music onboard Lightest weight of all models Longer battery life Cons Must keep iPhone nearby Only comes in aluminum Does not support Family Setup The Apple Watch GPS (Wi-Fi only) model is the least expensive and lightest weight of all models in the line. You get a longer battery life, but that`s because it doesn`t have a cellular chip. It requires an iPhone for daily use. Free bird Apple Watch GPS + Cellular From $329 at Apple Pros Apple Music streaming support Aluminum and Stainless Steel models Leave your iPhone at home All models come with 16GB-32GB of storage Works with Family Setup Cons More expensive model No version for Series 3 The GPS + Cellular model allows streaming of Apple Music and the Podcasts app. Series 4 has 16GB of storage, while the SE, Series 5, and 6 include 32GB of storage, and you can get all versions (except SE) in aluminum, stainless steel, and titanium. Series 6...