2-stroke Cons
Mixing gas & oil
Many manufacturers no longer produce them
Wheel-spin (OK, power bands aren't always fun)
4-stroke Pros
Smooth power delivery
Everybody has one
The don't sound like a sewing machine
4-stroke Cons
Slightly more expensive to purchase than 2-strokes
More expensive to maintain (can you say "VALVE JOBS"?)
Heavier than 2-strokes
Truth be told, it's hard to go wrong with either one. Pick your poison and let's move on.
What Size Engine
Bigger is better, right? Not always. It's a personal thing. As I said, I chose a 125 2-stroke and I'm definitely underpowered. I'm 6'3" and weigh about 215 pounds. My suspension has not been modified. Was I crazy when I decided on this bike? Not at all.
I'm 49 years old and my best racing days are behind me. I have to get up and go to work each day in order for my family to eat. I don't want a bike that far exceeds my riding ability. Believe me, I can get into plenty of trouble on the 125.
Besides, I think I ride better on the smaller bike because I have to concentrate. If I don't hit the turn just right, I won't have enough speed to clear the doubles. If I had my way, we'd ALL be on 125 smokers.
When considering what size best fits your need, keep in mind that a 125 2-stroke and a 250 4-stroke are basically comparable bikes. Likewise with the 250 2-stroke and the 450 4-stroke.