I Prefer Two-Strokes
You get more power to weight ratio and more power period per cc with a 2-stroke compared to 4-strokes.
They are a simpler engine which makes for easier and cheaper maintenance.
They still have a lower center of gravity which makes for better handling, and are still lighter all around.
About the only con I think the 2-strokes have besides the fact that in the past 4-strokes were allowed double the cc's in the 125 class and almost double the cc's in the 250 class (450's), is the grip / traction 4 strokes get due to their combustion stroke being half as often as the 2-stroke. It allows for gription similar to how anti-lock brakes work but backwards.
This helps the 4 strokes off the line at slow speeds, out of real tight turns, and in real dry turns. But on a good prepped track, a good rider on a 250 2-stroke will win hands down against a 250 4-stroke! Any day! And will compete even with a 450.
I mean, if 4-strokes are so fast, why did they allow 250f's to race with 125's and 450f's to race with 250's? A 150f has 25 more cc's than a 125 2 stroke and it is considered a mini bike for kids! Where the 125 2 stroke is a big bike! That is pathetic!
All in all it comes down to the rider. The machine helps a bit but that can also come down to how much money is put into the bike. But over all if you compare same size engines (cc's) with experienced riders on both, the 2-stroke will have the advantage.
And as far as pollution, I think our government should look into doing something about pollution from semis on the roads and all cars for that matter before dirt bikes. I doubt the difference in all dirt bikes being 4-strokes as opposed to 2-strokes will effect our pollution in even a tenth of a percent.
I ride and race a 2006 RM 250 with the only mods being an FMF Fatty pipe and a smaller main jet. This past season I marked into the 250 and Open Pro classes in the SRAC in Colorado.
I have raced on a 07 KXF 450 and I still know I would be faster on my 2-stroke and have no desire to fall into the con of the 4-joke ways, I mean stroke. d:-p