I assume that you already know the difference between 2 and 4 in terms of basic functions...

e.g. Like 2 cycle versus 4 cycle engines

A 4 stroke engine has a nicer sound and you'll hear everyone say "it can turn a bigger prop".

That is somewhat true...

However compare the output power of a .70 2 stroke engine verus a .70 4 stroker and you'll find that the 2 stroker engine is far more powerful.

The reason for this is that the 2 stroke engine ignites the fuel on every movement ( compression ) stroke of the cylinder. The 4 stroker, does it every other time.

As a result the 2 strokers build higher RPM's, but they feature ligher cylinders and less moving mass.

Conversely the 4 stroker uses a larger cylinder with more mass to help carry the engine through the non firing cycle.

The 4 stroker then tends to burn less fuel too. They also sound much nicer, more like a real plane engine.

So let's say you are looking at a .60 sized plane, and the instructions call for at most a .60 sized 2 stroke engine.

You could put in a .60 to .70 2 stroker or to get the equivalent output you'll need about a .91 ( yes this is inexact ) four stroker.

The four stroke engines cost more because they are more complex.

I've had about equally good luck with 2C and 4C engines.

Usually 2 stroke engines are used on "smaller" planes. As you move up fuel consumption rises quickly on the 2 strokers, making the 4 strokers a bit cheaper in the long run.

I typically go to 4 stroke engines in planes around the .90 and above size ( with a few exceptions ), and use 2 stroke engines on planes smaller than that...

Between .70 and .90 it's a real toss up for me.