That’s exactly what I did. I love the cell version when it works. It seems to require a VERY strong signal. The speed and battery life are amazing. I’m hoping 4.1 really helps in the signal department.
Absolutely it will be. In terms of performance of the upgrade processor and battery life, it's extended across the board with 50 M water resistance, brighter display, GPS, LTE, etc.
The only real similarity between the first generation Apple Watch and Series 3 is the physical design, but everything internally has almost been upgraded.