Which would you trade for

KX250, cause perimeter frame!

yz125, cause 125!

Your post isn't very clear on how much riding you actually do.  But from what I am gathering, not much.  If that's the case, the 125.     

The yz125 is one of the most fun machines there is but kind of likes to be run at a higher rpm and needs a bit of clutch and gas to climb or in softer conditions. The kids would love it and it really teaches good techniques  with clutch and gas which is a huge advantage for riding all bikes in their future. The 250 you can ride lower rpm and less clutch and it can get you out of situations easier as far as hills or mud goes on trails etc.  Both are super easy to work on. 

Yz125 more parts available for them new and aftermarket unless it's in worse condition than the 250. 

My brain says the 250 but my heart says the 125. I had an 01 YZ125 and loved it.

Is this even a question....

The 125....

Rule number one, don’t ask for any opinion on Vital ! Worthless

This reason alone! The KX would be a fun one to have BUT parts for older KX’s (especially 2-strokes!) are getting pretty hard to come by! The YZ will still have a reasonably good parts availability considering it’s over 20 years old! Plus…125’s are just COOL! 🤪 👍❗️

My '01 YZ125 was the bike I call my favorite of all time. I had 7 YZ125s, but I had the '01 working the best. 

The 125 will be more fun.  On or off a track.

Honestly, if you’ve still got the desire to ride a 125…

Then there’s idea of having a Bubba bike…

Hmmmmm…

I’d flip a coin or buy both!😂

125 for this reason, plus 125s are a blast to ride.

 imo:  185lbs or less = 125.....have fun

Honestly, I would say neither. 

These bikes are over 20 years old, and most examples haven't had the best up keep.  If you plan on riding something like this for 4 hours a days, then you can expect to put a bunch of work and money into it.  Be prepared to do a top end and carb rebuild at the very least in addition to routine maintenance. 

KX...you'll find the 125 all day online..

Post pics! 
 

those forks like 95-110mm oil height and change the oil in them often 

I have some power valves for a 99 if you ever need them as well as some neat cartridge seals for the forks that I never got the chance to use (bought them from @Bruce372 on here years ago)  

The YZ125's are amazing (profile pic), but the 01's were not as good (in my opinion). If it was an 05+ no question, get the YZ, but the steel framed YZ's were no where near the greatness as the aluminum framed 05's and newer. But the Perimeter framed KX's were the cats a$$.  

Ok, working on a 1990 KX 125. Suprising what is available for replacement / wear items from Kawasaki. Aftermarket is awesome for many things. There were just a few items that I had to resort to ebay. 250 probably has more torque down low. Putting with the kids without having to get the rpms way up should be better with the 250. Take a stand and ride. 

I'd spring for the YZ125. Easier to work on, parts are everywhere and the bike is still being made. Cheaper to rebuild and they're silly amounts of fun. I've been having way more fun riding my 04 CR125 lately than my KX450.

Unless it revs up on its own etc, the crank seals r good.  Just do the basics clean power valve,top end,  grease linkage, check reeds, cables, sprocket bolts., brakes. Etc And run it.  If your traveling 300 miles to race a full gate of 125,s at a big race. That’s where you go nuts.  I personally would’ve looked for a 04 or 05-07 cr125. Honda was weird about longevity testing 1990-2007 . There built a million times better than anything.  I haven’t been passed in 2 yrs at the local track on my 04 handles better than any 24 & it’s a blast to ride. 

Only a sociopath enjoy riding a 125. 

You're either starting dead last or you need to move up to the novice class.

Your crank seals won't show a compress loss unless they aren't in the motor at all. Just me but that motor would get a new top end and main seals. A 99 is now 25 years old and that stuff dries up and get hard. It could be a money pit unless you have a passion for old YZ125s. I see nothing wrong with that, I'm kind of a sucker for projects.