1. Generally, yes a good 4-stroke will give better throttle response than a piped 2-stroke. Also, the 4-stroke will have more low-RPM torque, which could be very useful. The 4-stroke will also probably be lighter.

2. The braking is almost entirely dependent on the prop you are using.

3. You can use a regulator (Cline or Iron Bay) to accomplish the same thing. Personally, I'd go with a Saito 1.25 with an Iron Bay regulator over the OS 120 pumped. You can place the tank anywhere you want with either setup.

4. Your vertical is going to be dependent on how you prop the plane, though I would tell you that the OS120 will be able to swing a larger prop than the OS91, thus allowing for more thrust.

5. I've never ever had a problem with an inverted 4-stroke.