I used the OS Max 70 Surpass, 91 Surpass, FS20 and FS60 in my models and tested the OS FS120 with pump, the older FS120, the Saito 90 twin and the Magnum XL FS52 on the bench.

They are a good choice if you want a light engine, which should operate around 10000 rpm. It is hard for a two stroke of the same weight to outperform a four stroke in this rpm range. Most likely the two stroke is heavier because he needs a bulky exhaust system and he will have problems to swing the same props with good transition. This can be solved if you use a "throttle muffler" for 3D applications with the two stroke but the aggressive response of a four stroke is difficult to beat.
If the desired rpm range is higher than 13000 the two strokes will probably do better.

A big advantage is that you don't need to think much about the exhaust system of a four stroke.
They perform very well with the simple stock muffler.

Normally there is no need to care about the displacement of your engine except you must observe competition rules.
If you just decide considering the power/weight ratio and other parameters which are important for your model you will often find that a four stroke is the best choice.

The reliability of every model engine depends on the user but I think the four strokes keep running a little longer if they have a few seconds of lean running in the air. This can be a deception because it is almost impossible to hear if they are too lean especially if they are airborne.
My FS70 and FS91 kept running until the tank was totally dry. There was no sign of getting lean when the tank was almost empty. They just switched off and that was it. Sometimes I wished that there would be at least a little sign of being low on fuel.

The run in process lasts a little longer than with the "ready out of the box" ABN 2-strokes because most of the four strokes are ringed engines but in return they last very long. The tendency to backfire will be reduced after a few running hours.

I would recommend a OS 52, 70 or 91 as a first four stroke. They are easy to start with a ordinary starter and it is impossible to hear bad things about them, exept the cracked valve springs a few years ago but this is not a problem if you by a new engine of the latest Surpass II series. The OS 4-strokes dont need much nitro. They run perfect with 10%.