BTW, some people have went from 2 stroke to 4 stroke and are now back on 2 strokes. Some are still on 2 strokes and most have switched to 4 stroke and are planning to stay there. Nobody is "right" - It's personal preference - don't let your friends tell you what to do.
You're wanting to ride the MX tracks or race? If you're wanting to race, you'll find the 250f more easily competitive.
If you've never ridden a 125 before, you'll be in for a surprise - perhaps both good and bad.
125s are some of the "funnest" bikes ever made (especially if you give them a bit of kick with an overbore. . . ;) . . .).
However, 125s have ZERO low end torque. Some 4 stroke guys would say that a 125 has no torque at all.
You have to learn to shift A LOT and use the clutch A LOT to keep up with the 4 strokes.
On the occasions when I've ridden a 250f or 450f, I always come away shocked at the "ridability". I may not like the bike set up, but the motors are always impressive. My friend rides our entire local track in 3rd gear on his KX250f - except for starting in 2nd and shifting to 2nd for one hairpin. I, on the other hand, sometimes shift to 1st for that hairpin and I go as high as 4th gear on other parts of the track. I have my hands full, so to speak. If I could, I'd have a 250f motor in my bike set up.
However, that friend I'm talking about was about the same speed as me on his previous bike - a 2000 CR125. Now that he has his 2005 KX250f, we're still about the same speed. If I'm in front of him, he's probably not gettting around - and vice versa.
I often come off a starting line filled with 250f's and even 450f's with a top 50% start - and last week I was top three and about to pass for 2nd when the 2nd place guy crashed right in front of me - I had to stop and wait for every other rider to go around before I could take off again.
Anyway, it's 95% bike, 5% rider, and few of us can ride any of the bikes anywhere near their potential.
Riders in the C and D classes are not what you'd call highly skilled across the board - and they're highly inconsistent - so your technique is very important.
Some may disagree with this statement, but it's my opinion that learning to ride a 125 well will teach you to be a better rider when/if you switch to a 4 stroke.
It's up to you - do you want to have a fun bike now and work on your technique or do you want to wait and save up.
Apparently you can afford a 125 now but can't afford a 4 stroke yet. . .
Personally, if I were in your position, I'd buy what I could afford now, ride it and work on my technique - and if you don't like it that much, save up over the winter, sell the 125 next spring and get a 4 stroke.
If you buy the 125 "low", next spring you might even be able to sell it for what you paid for it and not take a loss.
Given the circumstance, if you're thinking of buying a 125 and then changing to 4 stroke next year - I'd probably be looking for a good condition bike 2001-2004 range - you might even find something for $1000-1300 in that year range - if it's in decent condition and runs good, you can't lose much at that price.
Shop around - craigslist, ebay (local bikes you can go look at), newspapers - most papers have classifieds online, so you can look within a couple hundred miles pretty easily.
My '02 RM didn't look very good because the graphics were peeling, etc. . ., but it was mechanically sound and I bought in in spring 05 for $1500. That would be like buying a 2005 bike now for $1500.
