Bully:
This is going to be long, but I think it will be educational for you . . . LOL
First, as Eric Gorr says, "Speed is 95% rider and 5% bike. Better make that 5% count."
The 250F's have about the same peak horsepower as a 125 2 stroke - i.e. stock 125s and 250f's have about 31-33 HP stock. However, the 250f's have a lot more torque on the bottom end and midrange. Most people find them much easier to ride because you don't have to shift as much and the power delivery is more forgiving.
However, the 125's are lighter and generally handle better than the 250f's - i.e quicker cornering. Further, most people think that the 2 strokes are more fun to ride because of the extreme "hit" of the powerband in the mid/upper midrange.
I prefer the 2 stroke because of the weight, ease and amount of maintenance and the fun factor.
The 250f definitely has an advantage over the 125 on the track - I find that the advantage is evident mainly on the start and while exiting corners. The 250f tends to pull quicker without wheelspin. On the 125 you have to feather the clutch and learn to be precise with the throttle.
Nevertheless, in my first season of racing last year on a 2002 RM125, I had four 2nd place finishes in my class(es) in about 12 total races. I was able to beat numerous 250f's in the course of those races.
Since speed is 95% rider, if you're on a 125, you will beat almost all of the guys who arenot as good as you are. If you're racing head to head with a guy of equal skill, you will have a lot of difficulty beating him on the 125 if he's on a 250f.
That said, having done it already, this is what I would do if I were you and I want to "make that 5% count" on a 2 stroke 125. (Note: again, at the beginner and novice level, the 125 will be fine and you can beat almost all 250f's whose riders are not as good as you, but the following is the ideal setup for a 2 stroke, IMHO).
I would buy one of the 125's and send the motor to Eric Gorr for a 144cc overbore kit. Eric is closing up shop in April for several months, so there is still time to get it to him.
Because of the inherent advantage of the 250f, the AMA is allowing amateurs to overbore their 2 stroke 125s to 144cc. Eric did my kit for $450 and shipped the motor back to me free since I pre-paid. I'm not sure what he's charging now - prices went up in 2006. His website says $475.
This includes boring the cylinder 4mm, having the cylinder re-plated, porting the cylinder to your preferred riding style, modifying the head to the compression you want (pump gas or race gas - race gas provides about 1 HP improvement). The kit also includes the modifications to the low end case, the oversize piston and ring, and re-assembly.
When I got my motor back from him, I bolted it in and it started on the 6th or 7th kick. It now starts 1st or 2nd kick.
As to how the 144 runs, it's incredible. A 144 with race gas will run 37-39 HP - read my following post:
http://www.dirtrider.net/forums3/showthread.php?t=126800
. . .and all the posts in this one regarding reliability of the 144:
http://www.dirtrider.net/forums3/showthread.php?t=124121
. . .and here are some others:
http://www.dirtrider.net/forums3/showthread.php?t=127695
http://www.dirtrider.net/forums3/showthread.php?t=127984