6250 was sold as a better interlock design with helical cut gears which were quiet and stronger. But I've heard it's not a true interlock system and honestly with a small machine why would you care to use it. T bird made the 155 with I think a 60' boom and a side by side interlock drum system which I have heard was a great machine except for line capacity. The 155, 6250, 120 are newer gt3's for ten times the money lol. I spent a day with Harv at bighorn a few years back and watched that 6250 work, nice machine but they claimed it was too small. I also got to tour the madill shop just after they bought t bird and they had just refurbished a nice 155(the only yarder at the whole facility).