Go for more watts for a steep hill for sure. Likely the difference is the controller amps, and you will need it to be more. Climbing hills, pedal like hell if you have to, to keep the motor going at 15 mph / 25 kph. Below that most hubmotors make all the watts into heat, and then you fry stuff. If a really steep hill comes along and you can only go 10 mph, then oddly, you want to back off throttle, and keep pedaling your guts out.
Best bet for really steep hills, that is above 7-10% grades, is special motors with slower windings. These slow motors need much more volts to get to the same speed, but can stand going slower up steep hills without making heat. But you won't get those specialty motors from V power, at least not on a normal ebay auction.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.