Weight of a 170 is machine specific. Franklin offered the 170 in various wheelbases with several axle sizes an numerous log arch options. I think the the short wheelbased cable skidders are around 19,000 lb and the longer wheel based grapple skidders weigh closer to 15 ton. I have a mid 90's 170 single arch grapple that was remanufactured in 2005 and it seems to be fairly heavy. I know it will out pull the Deere 648 GIII I had.
I am not sure what a 405 weighs, but I think it will perform more like a 540 Deere or 240 TImberjack. Meaning quick, manuverable, and easier on fuel.
Why not consider a Franklin 560? It fits in right between the two you have considered.
I think your decision should be based on the task at hand. Is it more important for the skidder to be agile or to pull a big hitch?