If you are happy with a two stroke engine there is absolutely no reason to add the complexity and expense of a four stroke engine.
This may be true, however, if you don't have any experience with the 4 stroke, how can you make the judgement on which you prefer. I know I couldn't until I flew a 4 stroke engine (OS 70 II). I wouldn't fly anything else now...that's just me.
Kinda like comparing between a 2-stroke dirt bike and a Harley. The dirt bike gives economical, high-reving bursts of power, coupled with more smoke and thirstier consumption, but u have to be in the "powerband" to harness the power of the engine. The Harley on the other hand is torquey right from idle,
Herein lies the truth to why 4 strokes (in my opinion) are better. They produce most of their usable power lower in the rpm range, as compared to 2 strokes. This is why they may seem to have more torque. They pull better at a lower rpm. This is a feature that lends itself to 3D flying type planes too. Slower flying than usual is what these planes do well. Like the Funtana. If you had the chance to fly the Funtana with either type of engine, you would probably not want to take the 4 stroke off when you were finished flying.
As for...
I want to buy the 2 stroke for no other reason than ecenomical reasons.
I would say buying an engine cause it's cheap, or cheaper may not be more economical. The initial expense of a 4 stroke may be more in some cases, but I believe your fuel savings would offset that difference in time.
Jim