To put the difference as simply as possible, a 2 stroke's porting is at best a compromise and is far from being ideal because, of necessity, it has to be symmetrical around BDC. This drastically reduces it's efficiency but it's made up for by the fact it fires every revolution. Because it's not effecient it uses fuel faster. A 4 stroke on the other hand has the opening and closing points for it's porting system tailored much more closely to what's really required. This makes it much more effecient in the way it controls the available pressure inside the cylinder.

However, a 4 stroke has a far lower potential maximum rev limit because of (mainly) valve float and flow limitations through the valves/ports. Below this point though it develops quite a lot of torque (higher average cylinder pressures or BMEP) which is useful for driving big props.

A 2 stroke doesn't have any mechanical limitation on porting so generally they're designed to be able to flow large volumes of mixture at much higher revs. Their compromise porting sacrifices lower rev torque but continues to give reasonable torque at high revs which in turn means high BHP to drive smaller props at high RPM.

So in general from an efficiency point of view, a 4 stroke is a better choice for a model needing a relatively large prop at lower revs (ranging from trainers, sports and scale). The downside is the added complexity, cost and maybe weight.

A 2 stroke is the better choice for a small, clean model that's intended to fly fast (pylon or similar).

In practise though, either type of engine will fly just about any average model equally well.