OK, So I did a side-by-side mow/leaf collection between the "F" and the Duraforce, both are gold pro models. The jet has been drilled out on the Duraforce, so no surging noted. The Duraforce definitely has more power, and even "snarls" a bit. The "F" doesn't seem as powerful, but d_mn, it sure runs smooth. Both are running on Stihl full synthetic mixed at 40 to 1. Which would you keep, and which would you sell to the son of a very close friend? Lastly, they are both in near perfect condition, so how much would you ask for each? BTW, the "F" is a '95 sold new in '97. the duraforce is a 2001, sold in 2002, and has a new coil and plug. I removed, disassembled and cleaned the carbs on both, and installed sharpened&balanced blades on both. A little help?