The power of all these kits is about the same. There are other kits with more power, but they're more expensive. The things to think about are how and where you want to fit the battery, the size of the battery (aH) for distance, where to fit the throttle and whether you can fit the supplied ebrakes.
Some bikes have hydraulic brakes or integrated brake changers. The latter can be accommodated with inline brake sensors, so look for a kit with them included. Fitting a throttle is tricky, and depends on what type of gear-changers you have. Thumb throttles can be fitted either side. You can't have a twist-grip throttle if you have twist-grip gears.
You should check your forks are 100mm wide if you want a front motor. Whether you fit a front or rear motor makes little difference at 250w. Both have advantages and disadvantages.
There's also middle motors (Chain or crank-drives) which can be better if you have a lot of steep hills to climb.
Whichever kit you go for, try and get one with a panel where you can set the level of pedal assistance. You get LED panels with 3 levels and LCD ones with 5 levels.