Which do you prefer and why? I just got an F2 which came with a 120/60. It feels pretty good but I'm thinking about 120/70 when the time comes to purchase new tires. What do you guys think?

 

120/60 is the stock size that the bike was designed for. With a 120/70 you'll be reducing contact patch slightly, and making the overall tire taller, so your speedo will be registering a lower speed than you are really going.

Miy bike was sold to me with a 120/70 on it, and it also was a lot closer to the fender than when I put a 120/60 on.

 

I went to the 120/70 from the stock 120/60 to lessen the impacts going over the concrete lips in driveways. It always felt like the rim was hitting the ground. It helped a bunch. Racers always used the 70 series because the "feel" was better. The traction patch was about the same area, but longer. This may translate into a more gradual breakaway in traction. The taller profile caused problems at race speed by grinding into the fender. In street trim, I still have the stock front fender with no major wear. You can buy racing front fenders with extra clearance. I also pulled the forks up 10mm to compensate for the increase in tire diameter. This keeps the fork angle close to stock.