I had 250f's before and loved them (450's too but not as much) and know they are much easier to ride and ride fast but have been having a lot of fun on the 125's but like the weather in New England I change pretty quick. Being a 50+ vet rider I can ride whatever I want in any of the age classes I race and most guys ride 450's I have never felt at a big disadvantage on the 125 but I have to be on the edge ALL the time and your right it is a lot of work... so now that I have grown up I figure I can move back up to a 250f.
I really don't need a cutting edge bike and quite frankly the 250f's of probably 06 and up are better than me so to find a low hr late model is attractive to me but new is nice which is why when I saw 13 YZF's for mid 5's I started to think about it.
As far as carbs go, My KTM 125/144/150 were a nightmare to jet and I needed to run race gas in them but my YZ after going up on the main jet has been perfect in all weather/temp/elevation with no race gas. My KTM's had to be rejetted for Southwick so rich or they would blow up (they did, thats how I know) and then rejeted for the next track. Yamaha, nothing, my 89 CR125 once jetted has been great everywhere, my sons 99 is perfect everywhere. I feel once you know how to jet it is easy and with a fuel screw on a 4 stroke I found it easy enough to adjust to track conditions.
Of course my friends who have a FI bike say there is no going back to a carb for them ever and they LOVE there bikes. I have a friend with a 13 RMZ 250, one with a CRF 250 and one with a 12 YZF and have beeen offered test rides on all of them, hoping to try a couple this Sunday and I will report back....stay tuned for a full ride report!