Very few 125's are 6 speed. I would go with a YZ125, its two stroke and will beat the YZF125. Small displacement engines like 125's and 250's are great in 2 stroke. But bigger displacement bike like 450's are best in 4 strokI would get a Yamaha over a Honda any day. Yamaha's are more reliable, their 2 strokes are faster and Honda's engine only last a few years before they start to turn to shit.
I bought an 06 Honda CRF450R in 08 for my 16th birthday and the linkage bearings broke and the swing arm privet rusted and I constantly found metal shavings in the gearbox, it also happened to my friends CRF250R. I bought an 06 WR450 about six months ago and have had no problems at all, they are better to ride to.
Another option is a KTM, they are fast and reliable as well.