Here is my take on it.

If you want SPEED, get a 2-stroke.

If you want POWER, get a 4-stroke.

Let's face it, there's a reason dirt-bikes use 2-strokes, and Harleys use 4-strokes

Personally, I have 2-stroke 46 - 52. Anything above that, I switch to 4-strokes (One big reason is for fuel economy)

The 4-strokes I have are 52's 70's 91's 110's and a 150. Anything above that, I go Gas