Apple Watch GPS vs GPS + Cellular

I'm new to the Apple Watch game, so please excuse my ignorance! I'm trying to figure out the difference between GPS and GPS + cellular models other than the price tag...If I'm understanding correctly- since I have AT&T and they support WiFi calling, I'll be able to send/receive calls on my GPS capabilities ONLY Apple Watch as long as I'm either connected to my phone or connected to WiFi yes? And since 99.9% of the time I have my phone on me and the other .1% of the time I'm near a WiFi signal, basically I would be just as happy with the GPS only model (probably happier since I saved all that money!)? I'm asking because I've come across a brand new Series 5 for $150...I don't know its capabilities yet but if everything I've asked is accurate, then it doesn't matter and I should definitely buy it, right?

Sorry for all the questions, please be nice since I'm a newbie and thank you in advance for your help!