For me it depends on the riding. Motocross, or what passes for MX with my abilities, is much easier on a 125. I love my 05 YZ125 and my 75 RM125 even more. On a nice open grasstrack, I'll take a bigbore anytime. The best bikes I've ridden lately on grasstracks were my Husky CR500 (a real blast) and my HL500 (just good fun though not real quick). Enduro and trailriding give me a 175/200cc 2-stroke anytime, but I suspect I'd enjoy a modern 250F too. And for dirt track, I think a 250 2-stroke, although my favourite dirt track bike in recent years was my RM400N. Hmm... have I covered all the bases?
If I could only have one vintage bike, it'd be my RM125.