If you have the non cellular GPS watch, try a test by shutting your iPhone down before you get to home WiFi so your watch has connectivity, (or some other pre-known WiFi) then connect Find my iPhone from another device or computer, and see where it shows your devices. The watch should be clearly at home and the iPhone still some distance away until you power it up. My experience is the accuracy of the watch GPS is pretty good, within the limits I would expect considering possible GPS signal attenuation while you are in a building.
Assisted GPS (tower triangulation) should not make a significant difference to accuracy, but it can improve time to calculate a fix, and if GPS is blocked by buildings it will still provide a better-than-nothing fix with somewhat lower accuracy.