The i7-8650U has a higher stock clock and turbo so performance will be a bit better.
Both units have a tdp of 15w and a configurable tdp power up of 25w.
I see them as essentially the same so far as battery life and heat level are concerned.
No doubt, the stronger one will cost more.
Battery life will likely be determined more by the display choices
I suppose if they are both in the same open system, then the i7 would be better. However, if they are in a thin laptop like the X1 Yoga Gen 3, I wonder if its cooling system is good enough to make the i7 still better.
If one cpu has a higher clock rate and is 5% faster, it will generate more heat while working, yes, but it will complete the task in less time.
These new gen processors are very efficient.
You always have the option in windows to adjust the maximum clock rate used when on battery.
Ditto for the brightness of the display which is the real power hog.
Just be aware that "U" model CPUs cannot sustain high clockspeeds for very long since doing so generates a lot of heat. Under a sustained load clockspeeds will likely max out somewhere between 2.6GHz and 2.9GHz due to thermal throttling.