My experience has been that the most important factor in climbing hills is gearing and not the motor. Anything with a NuVinci hub climbs like a mountain goat. You may want to look at one of the Stanton kits that uses a NuVinci.
From my experience, most engine are very similar. Some are a little better than others, but if someone brags that their engines are way better than others... they are full of BS!
Could go 4 stroke. More cash though. The, well one of my favorite advantages of MBs over scooters is when extra power or speed is needed you have the peddles.
Is a hard question to answer. Personal choice and apples and oranges sort of deal. Would test ride both if possible.